// Copyright 2011 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "src/heap/spaces.h"

#include <utility>

#include "src/base/bits.h"
#include "src/base/macros.h"
#include "src/base/platform/semaphore.h"
#include "src/base/template-utils.h"
#include "src/counters.h"
#include "src/heap/array-buffer-tracker.h"
#include "src/heap/concurrent-marking.h"
#include "src/heap/gc-tracer.h"
#include "src/heap/heap-controller.h"
#include "src/heap/incremental-marking-inl.h"
#include "src/heap/mark-compact.h"
#include "src/heap/remembered-set.h"
#include "src/heap/slot-set.h"
#include "src/heap/sweeper.h"
#include "src/msan.h"
#include "src/objects-inl.h"
#include "src/objects/free-space-inl.h"
#include "src/objects/js-array-buffer-inl.h"
#include "src/objects/js-array-inl.h"
#include "src/ostreams.h"
#include "src/snapshot/snapshot.h"
#include "src/v8.h"
#include "src/vm-state-inl.h"

namespace v8 {
namespace internal {

    // These checks are here to ensure that the lower 32 bits of any real heap
    // object can't overlap with the lower 32 bits of cleared weak reference value
    // and therefore it's enough to compare only the lower 32 bits of a MaybeObject
    // in order to figure out if it's a cleared weak reference or not.
    STATIC_ASSERT(kClearedWeakHeapObjectLower32 > 0);
    STATIC_ASSERT(kClearedWeakHeapObjectLower32 < Page::kHeaderSize);
    STATIC_ASSERT(kClearedWeakHeapObjectLower32 < LargePage::kHeaderSize);

    // ----------------------------------------------------------------------------
    // HeapObjectIterator

    HeapObjectIterator::HeapObjectIterator(PagedSpace* space)
        : cur_addr_(kNullAddress)
        , cur_end_(kNullAddress)
        , space_(space)
        , page_range_(space->first_page(), nullptr)
        , current_page_(page_range_.begin())
    {
    }

    HeapObjectIterator::HeapObjectIterator(Page* page)
        : cur_addr_(kNullAddress)
        , cur_end_(kNullAddress)
        , space_(reinterpret_cast<PagedSpace*>(page->owner()))
        , page_range_(page)
        , current_page_(page_range_.begin())
    {
#ifdef DEBUG
        Space* owner = page->owner();
        DCHECK(owner == page->heap()->old_space() || owner == page->heap()->map_space() || owner == page->heap()->code_space() || owner == page->heap()->read_only_space());
#endif // DEBUG
    }

    // We have hit the end of the page and should advance to the next block of
    // objects.  This happens at the end of the page.
    bool HeapObjectIterator::AdvanceToNextPage()
    {
        DCHECK_EQ(cur_addr_, cur_end_);
        if (current_page_ == page_range_.end())
            return false;
        Page* cur_page = *(current_page_++);
        Heap* heap = space_->heap();

        heap->mark_compact_collector()->sweeper()->EnsurePageIsIterable(cur_page);
#ifdef ENABLE_MINOR_MC
        if (cur_page->IsFlagSet(Page::SWEEP_TO_ITERATE))
            heap->minor_mark_compact_collector()->MakeIterable(
                cur_page, MarkingTreatmentMode::CLEAR,
                FreeSpaceTreatmentMode::IGNORE_FREE_SPACE);
#else
        DCHECK(!cur_page->IsFlagSet(Page::SWEEP_TO_ITERATE));
#endif // ENABLE_MINOR_MC
        cur_addr_ = cur_page->area_start();
        cur_end_ = cur_page->area_end();
        DCHECK(cur_page->SweepingDone());
        return true;
    }

    PauseAllocationObserversScope::PauseAllocationObserversScope(Heap* heap)
        : heap_(heap)
    {
        DCHECK_EQ(heap->gc_state(), Heap::NOT_IN_GC);

        for (SpaceIterator it(heap_); it.has_next();) {
            it.next()->PauseAllocationObservers();
        }
    }

    PauseAllocationObserversScope::~PauseAllocationObserversScope()
    {
        for (SpaceIterator it(heap_); it.has_next();) {
            it.next()->ResumeAllocationObservers();
        }
    }

    static base::LazyInstance<CodeRangeAddressHint>::type code_range_address_hint = LAZY_INSTANCE_INITIALIZER;

    Address CodeRangeAddressHint::GetAddressHint(size_t code_range_size)
    {
        base::MutexGuard guard(&mutex_);
        auto it = recently_freed_.find(code_range_size);
        if (it == recently_freed_.end() || it->second.empty()) {
            return reinterpret_cast<Address>(GetRandomMmapAddr());
        }
        Address result = it->second.back();
        it->second.pop_back();
        return result;
    }

    void CodeRangeAddressHint::NotifyFreedCodeRange(Address code_range_start,
        size_t code_range_size)
    {
        base::MutexGuard guard(&mutex_);
        recently_freed_[code_range_size].push_back(code_range_start);
    }

    // -----------------------------------------------------------------------------
    // MemoryAllocator
    //

    MemoryAllocator::MemoryAllocator(Isolate* isolate, size_t capacity,
        size_t code_range_size)
        : isolate_(isolate)
        , data_page_allocator_(isolate->page_allocator())
        , code_page_allocator_(nullptr)
        , capacity_(RoundUp(capacity, Page::kPageSize))
        , size_(0)
        , size_executable_(0)
        , lowest_ever_allocated_(static_cast<Address>(-1ll))
        , highest_ever_allocated_(kNullAddress)
        , unmapper_(isolate->heap(), this)
    {
        InitializeCodePageAllocator(data_page_allocator_, code_range_size);
    }

    void MemoryAllocator::InitializeCodePageAllocator(
        v8::PageAllocator* page_allocator, size_t requested)
    {
        DCHECK_NULL(code_page_allocator_instance_.get());

        code_page_allocator_ = page_allocator;

        if (requested == 0) {
            if (!kRequiresCodeRange)
                return;
            // When a target requires the code range feature, we put all code objects
            // in a kMaximalCodeRangeSize range of virtual address space, so that
            // they can call each other with near calls.
            requested = kMaximalCodeRangeSize;
        } else if (requested <= kMinimumCodeRangeSize) {
            requested = kMinimumCodeRangeSize;
        }

        const size_t reserved_area = kReservedCodeRangePages * MemoryAllocator::GetCommitPageSize();
        if (requested < (kMaximalCodeRangeSize - reserved_area)) {
            requested += RoundUp(reserved_area, MemoryChunk::kPageSize);
            // Fullfilling both reserved pages requirement and huge code area
            // alignments is not supported (requires re-implementation).
            DCHECK_LE(kMinExpectedOSPageSize, page_allocator->AllocatePageSize());
        }
        DCHECK(!kRequiresCodeRange || requested <= kMaximalCodeRangeSize);

        Address hint = RoundDown(code_range_address_hint.Pointer()->GetAddressHint(requested),
            page_allocator->AllocatePageSize());
        VirtualMemory reservation(
            page_allocator, requested, reinterpret_cast<void*>(hint),
            Max(kMinExpectedOSPageSize, page_allocator->AllocatePageSize()));
        if (!reservation.IsReserved()) {
            V8::FatalProcessOutOfMemory(isolate_,
                "CodeRange setup: allocate virtual memory");
        }
        code_range_ = reservation.region();

        // We are sure that we have mapped a block of requested addresses.
        DCHECK_GE(reservation.size(), requested);
        Address base = reservation.address();

        // On some platforms, specifically Win64, we need to reserve some pages at
        // the beginning of an executable space. See
        //   https://cs.chromium.org/chromium/src/components/crash/content/
        //     app/crashpad_win.cc?rcl=fd680447881449fba2edcf0589320e7253719212&l=204
        // for details.
        if (reserved_area > 0) {
            if (!reservation.SetPermissions(base, reserved_area,
                    PageAllocator::kReadWrite))
                V8::FatalProcessOutOfMemory(isolate_, "CodeRange setup: set permissions");

            base += reserved_area;
        }
        Address aligned_base = RoundUp(base, MemoryChunk::kAlignment);
        size_t size = RoundDown(reservation.size() - (aligned_base - base) - reserved_area,
            MemoryChunk::kPageSize);
        DCHECK(IsAligned(aligned_base, kMinExpectedOSPageSize));

        LOG(isolate_,
            NewEvent("CodeRange", reinterpret_cast<void*>(reservation.address()),
                requested));

        heap_reservation_.TakeControl(&reservation);
        code_page_allocator_instance_ = base::make_unique<base::BoundedPageAllocator>(
            page_allocator, aligned_base, size,
            static_cast<size_t>(MemoryChunk::kAlignment));
        code_page_allocator_ = code_page_allocator_instance_.get();
    }

    void MemoryAllocator::TearDown()
    {
        unmapper()->TearDown();

        // Check that spaces were torn down before MemoryAllocator.
        DCHECK_EQ(size_, 0u);
        // TODO(gc) this will be true again when we fix FreeMemory.
        // DCHECK_EQ(0, size_executable_);
        capacity_ = 0;

        if (last_chunk_.IsReserved()) {
            last_chunk_.Free();
        }

        if (code_page_allocator_instance_.get()) {
            DCHECK(!code_range_.is_empty());
            code_range_address_hint.Pointer()->NotifyFreedCodeRange(code_range_.begin(),
                code_range_.size());
            code_range_ = base::AddressRegion();
            code_page_allocator_instance_.reset();
        }
        code_page_allocator_ = nullptr;
        data_page_allocator_ = nullptr;
    }

    class MemoryAllocator::Unmapper::UnmapFreeMemoryTask : public CancelableTask {
    public:
        explicit UnmapFreeMemoryTask(Isolate* isolate, Unmapper* unmapper)
            : CancelableTask(isolate)
            , unmapper_(unmapper)
            , tracer_(isolate->heap()->tracer())
        {
        }

    private:
        void RunInternal() override
        {
            TRACE_BACKGROUND_GC(tracer_,
                GCTracer::BackgroundScope::BACKGROUND_UNMAPPER);
            unmapper_->PerformFreeMemoryOnQueuedChunks<FreeMode::kUncommitPooled>();
            unmapper_->active_unmapping_tasks_--;
            unmapper_->pending_unmapping_tasks_semaphore_.Signal();
            if (FLAG_trace_unmapper) {
                PrintIsolate(unmapper_->heap_->isolate(),
                    "UnmapFreeMemoryTask Done: id=%" PRIu64 "\n", id());
            }
        }

        Unmapper* const unmapper_;
        GCTracer* const tracer_;
        DISALLOW_COPY_AND_ASSIGN(UnmapFreeMemoryTask);
    };

    void MemoryAllocator::Unmapper::FreeQueuedChunks()
    {
        if (!heap_->IsTearingDown() && FLAG_concurrent_sweeping) {
            if (!MakeRoomForNewTasks()) {
                // kMaxUnmapperTasks are already running. Avoid creating any more.
                if (FLAG_trace_unmapper) {
                    PrintIsolate(heap_->isolate(),
                        "Unmapper::FreeQueuedChunks: reached task limit (%d)\n",
                        kMaxUnmapperTasks);
                }
                return;
            }
            auto task = base::make_unique<UnmapFreeMemoryTask>(heap_->isolate(), this);
            if (FLAG_trace_unmapper) {
                PrintIsolate(heap_->isolate(),
                    "Unmapper::FreeQueuedChunks: new task id=%" PRIu64 "\n",
                    task->id());
            }
            DCHECK_LT(pending_unmapping_tasks_, kMaxUnmapperTasks);
            DCHECK_LE(active_unmapping_tasks_, pending_unmapping_tasks_);
            DCHECK_GE(active_unmapping_tasks_, 0);
            active_unmapping_tasks_++;
            task_ids_[pending_unmapping_tasks_++] = task->id();
            V8::GetCurrentPlatform()->CallOnWorkerThread(std::move(task));
        } else {
            PerformFreeMemoryOnQueuedChunks<FreeMode::kUncommitPooled>();
        }
    }

    void MemoryAllocator::Unmapper::CancelAndWaitForPendingTasks()
    {
        for (int i = 0; i < pending_unmapping_tasks_; i++) {
            if (heap_->isolate()->cancelable_task_manager()->TryAbort(task_ids_[i]) != TryAbortResult::kTaskAborted) {
                pending_unmapping_tasks_semaphore_.Wait();
            }
        }
        pending_unmapping_tasks_ = 0;
        active_unmapping_tasks_ = 0;

        if (FLAG_trace_unmapper) {
            PrintIsolate(
                heap_->isolate(),
                "Unmapper::CancelAndWaitForPendingTasks: no tasks remaining\n");
        }
    }

    void MemoryAllocator::Unmapper::PrepareForGC()
    {
        // Free non-regular chunks because they cannot be re-used.
        PerformFreeMemoryOnQueuedNonRegularChunks();
    }

    void MemoryAllocator::Unmapper::EnsureUnmappingCompleted()
    {
        CancelAndWaitForPendingTasks();
        PerformFreeMemoryOnQueuedChunks<FreeMode::kReleasePooled>();
    }

    bool MemoryAllocator::Unmapper::MakeRoomForNewTasks()
    {
        DCHECK_LE(pending_unmapping_tasks_, kMaxUnmapperTasks);

        if (active_unmapping_tasks_ == 0 && pending_unmapping_tasks_ > 0) {
            // All previous unmapping tasks have been run to completion.
            // Finalize those tasks to make room for new ones.
            CancelAndWaitForPendingTasks();
        }
        return pending_unmapping_tasks_ != kMaxUnmapperTasks;
    }

    void MemoryAllocator::Unmapper::PerformFreeMemoryOnQueuedNonRegularChunks()
    {
        MemoryChunk* chunk = nullptr;
        while ((chunk = GetMemoryChunkSafe<kNonRegular>()) != nullptr) {
            allocator_->PerformFreeMemory(chunk);
        }
    }

    template <MemoryAllocator::Unmapper::FreeMode mode>
    void MemoryAllocator::Unmapper::PerformFreeMemoryOnQueuedChunks()
    {
        MemoryChunk* chunk = nullptr;
        if (FLAG_trace_unmapper) {
            PrintIsolate(
                heap_->isolate(),
                "Unmapper::PerformFreeMemoryOnQueuedChunks: %d queued chunks\n",
                NumberOfChunks());
        }
        // Regular chunks.
        while ((chunk = GetMemoryChunkSafe<kRegular>()) != nullptr) {
            bool pooled = chunk->IsFlagSet(MemoryChunk::POOLED);
            allocator_->PerformFreeMemory(chunk);
            if (pooled)
                AddMemoryChunkSafe<kPooled>(chunk);
        }
        if (mode == MemoryAllocator::Unmapper::FreeMode::kReleasePooled) {
            // The previous loop uncommitted any pages marked as pooled and added them
            // to the pooled list. In case of kReleasePooled we need to free them
            // though.
            while ((chunk = GetMemoryChunkSafe<kPooled>()) != nullptr) {
                allocator_->Free<MemoryAllocator::kAlreadyPooled>(chunk);
            }
        }
        PerformFreeMemoryOnQueuedNonRegularChunks();
    }

    void MemoryAllocator::Unmapper::TearDown()
    {
        CHECK_EQ(0, pending_unmapping_tasks_);
        PerformFreeMemoryOnQueuedChunks<FreeMode::kReleasePooled>();
        for (int i = 0; i < kNumberOfChunkQueues; i++) {
            DCHECK(chunks_[i].empty());
        }
    }

    size_t MemoryAllocator::Unmapper::NumberOfCommittedChunks()
    {
        base::MutexGuard guard(&mutex_);
        return chunks_[kRegular].size() + chunks_[kNonRegular].size();
    }

    int MemoryAllocator::Unmapper::NumberOfChunks()
    {
        base::MutexGuard guard(&mutex_);
        size_t result = 0;
        for (int i = 0; i < kNumberOfChunkQueues; i++) {
            result += chunks_[i].size();
        }
        return static_cast<int>(result);
    }

    size_t MemoryAllocator::Unmapper::CommittedBufferedMemory()
    {
        base::MutexGuard guard(&mutex_);

        size_t sum = 0;
        // kPooled chunks are already uncommited. We only have to account for
        // kRegular and kNonRegular chunks.
        for (auto& chunk : chunks_[kRegular]) {
            sum += chunk->size();
        }
        for (auto& chunk : chunks_[kNonRegular]) {
            sum += chunk->size();
        }
        return sum;
    }

    bool MemoryAllocator::CommitMemory(VirtualMemory* reservation)
    {
        Address base = reservation->address();
        size_t size = reservation->size();
        if (!reservation->SetPermissions(base, size, PageAllocator::kReadWrite)) {
            return false;
        }
        UpdateAllocatedSpaceLimits(base, base + size);
        isolate_->counters()->memory_allocated()->Increment(static_cast<int>(size));
        return true;
    }

    bool MemoryAllocator::UncommitMemory(VirtualMemory* reservation)
    {
        size_t size = reservation->size();
        if (!reservation->SetPermissions(reservation->address(), size,
                PageAllocator::kNoAccess)) {
            return false;
        }
        isolate_->counters()->memory_allocated()->Decrement(static_cast<int>(size));
        return true;
    }

    void MemoryAllocator::FreeMemory(v8::PageAllocator* page_allocator,
        Address base, size_t size)
    {
        CHECK(FreePages(page_allocator, reinterpret_cast<void*>(base), size));
    }

    Address MemoryAllocator::AllocateAlignedMemory(
        size_t reserve_size, size_t commit_size, size_t alignment,
        Executability executable, void* hint, VirtualMemory* controller)
    {
        v8::PageAllocator* page_allocator = this->page_allocator(executable);
        DCHECK(commit_size <= reserve_size);
        VirtualMemory reservation(page_allocator, reserve_size, hint, alignment);
        if (!reservation.IsReserved())
            return kNullAddress;
        Address base = reservation.address();
        size_ += reservation.size();

        if (executable == EXECUTABLE) {
            if (!CommitExecutableMemory(&reservation, base, commit_size,
                    reserve_size)) {
                base = kNullAddress;
            }
        } else {
            if (reservation.SetPermissions(base, commit_size,
                    PageAllocator::kReadWrite)) {
                UpdateAllocatedSpaceLimits(base, base + commit_size);
            } else {
                base = kNullAddress;
            }
        }

        if (base == kNullAddress) {
            // Failed to commit the body. Free the mapping and any partially committed
            // regions inside it.
            reservation.Free();
            size_ -= reserve_size;
            return kNullAddress;
        }

        controller->TakeControl(&reservation);
        return base;
    }

    void MemoryChunk::DiscardUnusedMemory(Address addr, size_t size)
    {
        base::AddressRegion memory_area = MemoryAllocator::ComputeDiscardMemoryArea(addr, size);
        if (memory_area.size() != 0) {
            MemoryAllocator* memory_allocator = heap_->memory_allocator();
            v8::PageAllocator* page_allocator = memory_allocator->page_allocator(executable());
            CHECK(page_allocator->DiscardSystemPages(
                reinterpret_cast<void*>(memory_area.begin()), memory_area.size()));
        }
    }

    size_t MemoryChunkLayout::CodePageGuardStartOffset()
    {
        // We are guarding code pages: the first OS page after the header
        // will be protected as non-writable.
        return ::RoundUp(Page::kHeaderSize, MemoryAllocator::GetCommitPageSize());
    }

    size_t MemoryChunkLayout::CodePageGuardSize()
    {
        return MemoryAllocator::GetCommitPageSize();
    }

    intptr_t MemoryChunkLayout::ObjectStartOffsetInCodePage()
    {
        // We are guarding code pages: the first OS page after the header
        // will be protected as non-writable.
        return CodePageGuardStartOffset() + CodePageGuardSize();
    }

    intptr_t MemoryChunkLayout::ObjectEndOffsetInCodePage()
    {
        // We are guarding code pages: the last OS page will be protected as
        // non-writable.
        return Page::kPageSize - static_cast<int>(MemoryAllocator::GetCommitPageSize());
    }

    size_t MemoryChunkLayout::AllocatableMemoryInCodePage()
    {
        size_t memory = ObjectEndOffsetInCodePage() - ObjectStartOffsetInCodePage();
        DCHECK_LE(kMaxRegularHeapObjectSize, memory);
        return memory;
    }

    intptr_t MemoryChunkLayout::ObjectStartOffsetInDataPage()
    {
        return RoundUp(MemoryChunk::kHeaderSize, kTaggedSize);
    }

    size_t MemoryChunkLayout::ObjectStartOffsetInMemoryChunk(
        AllocationSpace space)
    {
        if (space == CODE_SPACE) {
            return ObjectStartOffsetInCodePage();
        }
        return ObjectStartOffsetInDataPage();
    }

    size_t MemoryChunkLayout::AllocatableMemoryInDataPage()
    {
        size_t memory = MemoryChunk::kPageSize - ObjectStartOffsetInDataPage();
        DCHECK_LE(kMaxRegularHeapObjectSize, memory);
        return memory;
    }

    size_t MemoryChunkLayout::AllocatableMemoryInMemoryChunk(
        AllocationSpace space)
    {
        if (space == CODE_SPACE) {
            return AllocatableMemoryInCodePage();
        }
        return AllocatableMemoryInDataPage();
    }

    Heap* MemoryChunk::synchronized_heap()
    {
        return reinterpret_cast<Heap*>(
            base::Acquire_Load(reinterpret_cast<base::AtomicWord*>(&heap_)));
    }

    void MemoryChunk::InitializationMemoryFence()
    {
        base::SeqCst_MemoryFence();
#ifdef THREAD_SANITIZER
        // Since TSAN does not process memory fences, we use the following annotation
        // to tell TSAN that there is no data race when emitting a
        // InitializationMemoryFence. Note that the other thread still needs to
        // perform MemoryChunk::synchronized_heap().
        base::Release_Store(reinterpret_cast<base::AtomicWord*>(&heap_),
            reinterpret_cast<base::AtomicWord>(heap_));
#endif
    }

    void MemoryChunk::DecrementWriteUnprotectCounterAndMaybeSetPermissions(
        PageAllocator::Permission permission)
    {
        DCHECK(permission == PageAllocator::kRead || permission == PageAllocator::kReadExecute);
        DCHECK(IsFlagSet(MemoryChunk::IS_EXECUTABLE));
        DCHECK(owner()->identity() == CODE_SPACE || owner()->identity() == CODE_LO_SPACE);
        // Decrementing the write_unprotect_counter_ and changing the page
        // protection mode has to be atomic.
        base::MutexGuard guard(page_protection_change_mutex_);
        if (write_unprotect_counter_ == 0) {
            // This is a corner case that may happen when we have a
            // CodeSpaceMemoryModificationScope open and this page was newly
            // added.
            return;
        }
        write_unprotect_counter_--;
        DCHECK_LT(write_unprotect_counter_, kMaxWriteUnprotectCounter);
        if (write_unprotect_counter_ == 0) {
            Address protect_start = address() + MemoryChunkLayout::ObjectStartOffsetInCodePage();
            size_t page_size = MemoryAllocator::GetCommitPageSize();
            DCHECK(IsAligned(protect_start, page_size));
            size_t protect_size = RoundUp(area_size(), page_size);
            CHECK(reservation_.SetPermissions(protect_start, protect_size, permission));
        }
    }

    void MemoryChunk::SetReadable()
    {
        DecrementWriteUnprotectCounterAndMaybeSetPermissions(PageAllocator::kRead);
    }

    void MemoryChunk::SetReadAndExecutable()
    {
        DCHECK(!FLAG_jitless);
        DecrementWriteUnprotectCounterAndMaybeSetPermissions(
            PageAllocator::kReadExecute);
    }

    void MemoryChunk::SetReadAndWritable()
    {
        DCHECK(IsFlagSet(MemoryChunk::IS_EXECUTABLE));
        DCHECK(owner()->identity() == CODE_SPACE || owner()->identity() == CODE_LO_SPACE);
        // Incrementing the write_unprotect_counter_ and changing the page
        // protection mode has to be atomic.
        base::MutexGuard guard(page_protection_change_mutex_);
        write_unprotect_counter_++;
        DCHECK_LE(write_unprotect_counter_, kMaxWriteUnprotectCounter);
        if (write_unprotect_counter_ == 1) {
            Address unprotect_start = address() + MemoryChunkLayout::ObjectStartOffsetInCodePage();
            size_t page_size = MemoryAllocator::GetCommitPageSize();
            DCHECK(IsAligned(unprotect_start, page_size));
            size_t unprotect_size = RoundUp(area_size(), page_size);
            CHECK(reservation_.SetPermissions(unprotect_start, unprotect_size,
                PageAllocator::kReadWrite));
        }
    }

    namespace {

        PageAllocator::Permission DefaultWritableCodePermissions()
        {
            return FLAG_jitless ? PageAllocator::kReadWrite
                                : PageAllocator::kReadWriteExecute;
        }

    } // namespace

    MemoryChunk* MemoryChunk::Initialize(Heap* heap, Address base, size_t size,
        Address area_start, Address area_end,
        Executability executable, Space* owner,
        VirtualMemory reservation)
    {
        MemoryChunk* chunk = FromAddress(base);

        DCHECK_EQ(base, chunk->address());

        chunk->heap_ = heap;
        chunk->size_ = size;
        chunk->header_sentinel_ = HeapObject::FromAddress(base).ptr();
        DCHECK(HasHeaderSentinel(area_start));
        chunk->area_start_ = area_start;
        chunk->area_end_ = area_end;
        chunk->flags_ = Flags(NO_FLAGS);
        chunk->set_owner(owner);
        chunk->InitializeReservedMemory();
        base::AsAtomicPointer::Release_Store(&chunk->slot_set_[OLD_TO_NEW], nullptr);
        base::AsAtomicPointer::Release_Store(&chunk->slot_set_[OLD_TO_OLD], nullptr);
        base::AsAtomicPointer::Release_Store(&chunk->typed_slot_set_[OLD_TO_NEW],
            nullptr);
        base::AsAtomicPointer::Release_Store(&chunk->typed_slot_set_[OLD_TO_OLD],
            nullptr);
        chunk->invalidated_slots_ = nullptr;
        chunk->skip_list_ = nullptr;
        chunk->progress_bar_ = 0;
        chunk->high_water_mark_ = static_cast<intptr_t>(area_start - base);
        chunk->set_concurrent_sweeping_state(kSweepingDone);
        chunk->page_protection_change_mutex_ = new base::Mutex();
        chunk->write_unprotect_counter_ = 0;
        chunk->mutex_ = new base::Mutex();
        chunk->allocated_bytes_ = chunk->area_size();
        chunk->wasted_memory_ = 0;
        chunk->young_generation_bitmap_ = nullptr;
        chunk->marking_bitmap_ = nullptr;
        chunk->local_tracker_ = nullptr;

        chunk->external_backing_store_bytes_[ExternalBackingStoreType::kArrayBuffer] = 0;
        chunk->external_backing_store_bytes_
            [ExternalBackingStoreType::kExternalString]
            = 0;

        for (int i = kFirstCategory; i < kNumberOfCategories; i++) {
            chunk->categories_[i] = nullptr;
        }

        chunk->AllocateMarkingBitmap();
        if (owner->identity() == RO_SPACE) {
            heap->incremental_marking()
                ->non_atomic_marking_state()
                ->bitmap(chunk)
                ->MarkAllBits();
        } else {
            heap->incremental_marking()->non_atomic_marking_state()->SetLiveBytes(chunk,
                0);
        }

        DCHECK_EQ(kFlagsOffset, OFFSET_OF(MemoryChunk, flags_));
        DCHECK_EQ(kHeapOffset, OFFSET_OF(MemoryChunk, heap_));
        DCHECK_EQ(kOwnerOffset, OFFSET_OF(MemoryChunk, owner_));

        if (executable == EXECUTABLE) {
            chunk->SetFlag(IS_EXECUTABLE);
            if (heap->write_protect_code_memory()) {
                chunk->write_unprotect_counter_ = heap->code_space_memory_modification_scope_depth();
            } else {
                size_t page_size = MemoryAllocator::GetCommitPageSize();
                DCHECK(IsAligned(area_start, page_size));
                size_t area_size = RoundUp(area_end - area_start, page_size);
                CHECK(reservation.SetPermissions(area_start, area_size,
                    DefaultWritableCodePermissions()));
            }
        }

        chunk->reservation_ = std::move(reservation);

        return chunk;
    }

    Page* PagedSpace::InitializePage(MemoryChunk* chunk, Executability executable)
    {
        Page* page = static_cast<Page*>(chunk);
        DCHECK_EQ(MemoryChunkLayout::AllocatableMemoryInMemoryChunk(
                      page->owner()->identity()),
            page->area_size());
        // Make sure that categories are initialized before freeing the area.
        page->ResetAllocatedBytes();
        page->SetOldGenerationPageFlags(heap()->incremental_marking()->IsMarking());
        page->AllocateFreeListCategories();
        page->InitializeFreeListCategories();
        page->list_node().Initialize();
        page->InitializationMemoryFence();
        return page;
    }

    Page* SemiSpace::InitializePage(MemoryChunk* chunk, Executability executable)
    {
        DCHECK_EQ(executable, Executability::NOT_EXECUTABLE);
        bool in_to_space = (id() != kFromSpace);
        chunk->SetFlag(in_to_space ? MemoryChunk::TO_PAGE : MemoryChunk::FROM_PAGE);
        Page* page = static_cast<Page*>(chunk);
        page->SetYoungGenerationPageFlags(heap()->incremental_marking()->IsMarking());
        page->AllocateLocalTracker();
        page->list_node().Initialize();
#ifdef ENABLE_MINOR_MC
        if (FLAG_minor_mc) {
            page->AllocateYoungGenerationBitmap();
            heap()
                ->minor_mark_compact_collector()
                ->non_atomic_marking_state()
                ->ClearLiveness(page);
        }
#endif // ENABLE_MINOR_MC
        page->InitializationMemoryFence();
        return page;
    }

    LargePage* LargePage::Initialize(Heap* heap, MemoryChunk* chunk,
        Executability executable)
    {
        if (executable && chunk->size() > LargePage::kMaxCodePageSize) {
            STATIC_ASSERT(LargePage::kMaxCodePageSize <= TypedSlotSet::kMaxOffset);
            FATAL("Code page is too large.");
        }

        MSAN_ALLOCATED_UNINITIALIZED_MEMORY(chunk->area_start(), chunk->area_size());

        // Initialize the sentinel value for each page boundary since the mutator
        // may initialize the object starting from its end.
        Address sentinel = chunk->address() + MemoryChunk::kHeaderSentinelOffset + MemoryChunk::kPageSize;
        while (sentinel < chunk->area_end()) {
            *reinterpret_cast<intptr_t*>(sentinel) = kNullAddress;
            sentinel += MemoryChunk::kPageSize;
        }

        LargePage* page = static_cast<LargePage*>(chunk);
        page->SetFlag(MemoryChunk::LARGE_PAGE);
        page->list_node().Initialize();
        return page;
    }

    void Page::AllocateFreeListCategories()
    {
        for (int i = kFirstCategory; i < kNumberOfCategories; i++) {
            categories_[i] = new FreeListCategory(
                reinterpret_cast<PagedSpace*>(owner())->free_list(), this);
        }
    }

    void Page::InitializeFreeListCategories()
    {
        for (int i = kFirstCategory; i < kNumberOfCategories; i++) {
            categories_[i]->Initialize(static_cast<FreeListCategoryType>(i));
        }
    }

    void Page::ReleaseFreeListCategories()
    {
        for (int i = kFirstCategory; i < kNumberOfCategories; i++) {
            if (categories_[i] != nullptr) {
                delete categories_[i];
                categories_[i] = nullptr;
            }
        }
    }

    Page* Page::ConvertNewToOld(Page* old_page)
    {
        DCHECK(old_page);
        DCHECK(old_page->InNewSpace());
        OldSpace* old_space = old_page->heap()->old_space();
        old_page->set_owner(old_space);
        old_page->SetFlags(0, static_cast<uintptr_t>(~0));
        Page* new_page = old_space->InitializePage(old_page, NOT_EXECUTABLE);
        old_space->AddPage(new_page);
        return new_page;
    }

    size_t MemoryChunk::CommittedPhysicalMemory()
    {
        if (!base::OS::HasLazyCommits() || owner()->identity() == LO_SPACE)
            return size();
        return high_water_mark_;
    }

    bool MemoryChunk::InOldSpace() const
    {
        return owner()->identity() == OLD_SPACE;
    }

    bool MemoryChunk::InLargeObjectSpace() const
    {
        return owner()->identity() == LO_SPACE;
    }

    MemoryChunk* MemoryAllocator::AllocateChunk(size_t reserve_area_size,
        size_t commit_area_size,
        Executability executable,
        Space* owner)
    {
        DCHECK_LE(commit_area_size, reserve_area_size);

        size_t chunk_size;
        Heap* heap = isolate_->heap();
        Address base = kNullAddress;
        VirtualMemory reservation;
        Address area_start = kNullAddress;
        Address area_end = kNullAddress;
        void* address_hint = AlignedAddress(heap->GetRandomMmapAddr(), MemoryChunk::kAlignment);

        //
        // MemoryChunk layout:
        //
        //             Executable
        // +----------------------------+<- base aligned with MemoryChunk::kAlignment
        // |           Header           |
        // +----------------------------+<- base + CodePageGuardStartOffset
        // |           Guard            |
        // +----------------------------+<- area_start_
        // |           Area             |
        // +----------------------------+<- area_end_ (area_start + commit_area_size)
        // |   Committed but not used   |
        // +----------------------------+<- aligned at OS page boundary
        // | Reserved but not committed |
        // +----------------------------+<- aligned at OS page boundary
        // |           Guard            |
        // +----------------------------+<- base + chunk_size
        //
        //           Non-executable
        // +----------------------------+<- base aligned with MemoryChunk::kAlignment
        // |          Header            |
        // +----------------------------+<- area_start_ (base + area_start_)
        // |           Area             |
        // +----------------------------+<- area_end_ (area_start + commit_area_size)
        // |  Committed but not used    |
        // +----------------------------+<- aligned at OS page boundary
        // | Reserved but not committed |
        // +----------------------------+<- base + chunk_size
        //

        if (executable == EXECUTABLE) {
            chunk_size = ::RoundUp(MemoryChunkLayout::ObjectStartOffsetInCodePage() + reserve_area_size + MemoryChunkLayout::CodePageGuardSize(),
                GetCommitPageSize());

            // Size of header (not executable) plus area (executable).
            size_t commit_size = ::RoundUp(
                MemoryChunkLayout::CodePageGuardStartOffset() + commit_area_size,
                GetCommitPageSize());
            base = AllocateAlignedMemory(chunk_size, commit_size, MemoryChunk::kAlignment,
                executable, address_hint, &reservation);
            if (base == kNullAddress)
                return nullptr;
            // Update executable memory size.
            size_executable_ += reservation.size();

            if (Heap::ShouldZapGarbage()) {
                ZapBlock(base, MemoryChunkLayout::CodePageGuardStartOffset(), kZapValue);
                ZapBlock(base + MemoryChunkLayout::ObjectStartOffsetInCodePage(),
                    commit_area_size, kZapValue);
            }

            area_start = base + MemoryChunkLayout::ObjectStartOffsetInCodePage();
            area_end = area_start + commit_area_size;
        } else {
            chunk_size = ::RoundUp(
                MemoryChunkLayout::ObjectStartOffsetInDataPage() + reserve_area_size,
                GetCommitPageSize());
            size_t commit_size = ::RoundUp(
                MemoryChunkLayout::ObjectStartOffsetInDataPage() + commit_area_size,
                GetCommitPageSize());
            base = AllocateAlignedMemory(chunk_size, commit_size, MemoryChunk::kAlignment,
                executable, address_hint, &reservation);

            if (base == kNullAddress)
                return nullptr;

            if (Heap::ShouldZapGarbage()) {
                ZapBlock(
                    base,
                    MemoryChunkLayout::ObjectStartOffsetInDataPage() + commit_area_size,
                    kZapValue);
            }

            area_start = base + MemoryChunkLayout::ObjectStartOffsetInDataPage();
            area_end = area_start + commit_area_size;
        }

        // Use chunk_size for statistics and callbacks because we assume that they
        // treat reserved but not-yet committed memory regions of chunks as allocated.
        isolate_->counters()->memory_allocated()->Increment(
            static_cast<int>(chunk_size));

        LOG(isolate_,
            NewEvent("MemoryChunk", reinterpret_cast<void*>(base), chunk_size));

        // We cannot use the last chunk in the address space because we would
        // overflow when comparing top and limit if this chunk is used for a
        // linear allocation area.
        if ((base + chunk_size) == 0u) {
            CHECK(!last_chunk_.IsReserved());
            last_chunk_.TakeControl(&reservation);
            UncommitMemory(&last_chunk_);
            size_ -= chunk_size;
            if (executable == EXECUTABLE) {
                size_executable_ -= chunk_size;
            }
            CHECK(last_chunk_.IsReserved());
            return AllocateChunk(reserve_area_size, commit_area_size, executable,
                owner);
        }

        MemoryChunk* chunk = MemoryChunk::Initialize(heap, base, chunk_size, area_start, area_end,
            executable, owner, std::move(reservation));

        if (chunk->executable())
            RegisterExecutableMemoryChunk(chunk);
        return chunk;
    }

    void MemoryChunk::SetOldGenerationPageFlags(bool is_marking)
    {
        if (is_marking) {
            SetFlag(MemoryChunk::POINTERS_TO_HERE_ARE_INTERESTING);
            SetFlag(MemoryChunk::POINTERS_FROM_HERE_ARE_INTERESTING);
            SetFlag(MemoryChunk::INCREMENTAL_MARKING);
        } else {
            ClearFlag(MemoryChunk::POINTERS_TO_HERE_ARE_INTERESTING);
            SetFlag(MemoryChunk::POINTERS_FROM_HERE_ARE_INTERESTING);
            ClearFlag(MemoryChunk::INCREMENTAL_MARKING);
        }
    }

    void MemoryChunk::SetYoungGenerationPageFlags(bool is_marking)
    {
        SetFlag(MemoryChunk::POINTERS_TO_HERE_ARE_INTERESTING);
        if (is_marking) {
            SetFlag(MemoryChunk::POINTERS_FROM_HERE_ARE_INTERESTING);
            SetFlag(MemoryChunk::INCREMENTAL_MARKING);
        } else {
            ClearFlag(MemoryChunk::POINTERS_FROM_HERE_ARE_INTERESTING);
            ClearFlag(MemoryChunk::INCREMENTAL_MARKING);
        }
    }

    void Page::ResetAllocatedBytes() { allocated_bytes_ = area_size(); }

    void Page::AllocateLocalTracker()
    {
        DCHECK_NULL(local_tracker_);
        local_tracker_ = new LocalArrayBufferTracker(this);
    }

    bool Page::contains_array_buffers()
    {
        return local_tracker_ != nullptr && !local_tracker_->IsEmpty();
    }

    void Page::ResetFreeListStatistics()
    {
        wasted_memory_ = 0;
    }

    size_t Page::AvailableInFreeList()
    {
        size_t sum = 0;
        ForAllFreeListCategories([&sum](FreeListCategory* category) {
            sum += category->available();
        });
        return sum;
    }

#ifdef DEBUG
    namespace {
        // Skips filler starting from the given filler until the end address.
        // Returns the first address after the skipped fillers.
        Address SkipFillers(HeapObject filler, Address end)
        {
            Address addr = filler->address();
            while (addr < end) {
                filler = HeapObject::FromAddress(addr);
                CHECK(filler->IsFiller());
                addr = filler->address() + filler->Size();
            }
            return addr;
        }
    } // anonymous namespace
#endif // DEBUG

    size_t Page::ShrinkToHighWaterMark()
    {
        // Shrinking only makes sense outside of the CodeRange, where we don't care
        // about address space fragmentation.
        VirtualMemory* reservation = reserved_memory();
        if (!reservation->IsReserved())
            return 0;

        // Shrink pages to high water mark. The water mark points either to a filler
        // or the area_end.
        HeapObject filler = HeapObject::FromAddress(HighWaterMark());
        if (filler->address() == area_end())
            return 0;
        CHECK(filler->IsFiller());
        // Ensure that no objects were allocated in [filler, area_end) region.
        DCHECK_EQ(area_end(), SkipFillers(filler, area_end()));
        // Ensure that no objects will be allocated on this page.
        DCHECK_EQ(0u, AvailableInFreeList());

        size_t unused = RoundDown(static_cast<size_t>(area_end() - filler->address()),
            MemoryAllocator::GetCommitPageSize());
        if (unused > 0) {
            DCHECK_EQ(0u, unused % MemoryAllocator::GetCommitPageSize());
            if (FLAG_trace_gc_verbose) {
                PrintIsolate(heap()->isolate(), "Shrinking page %p: end %p -> %p\n",
                    reinterpret_cast<void*>(this),
                    reinterpret_cast<void*>(area_end()),
                    reinterpret_cast<void*>(area_end() - unused));
            }
            heap()->CreateFillerObjectAt(
                filler->address(),
                static_cast<int>(area_end() - filler->address() - unused),
                ClearRecordedSlots::kNo);
            heap()->memory_allocator()->PartialFreeMemory(
                this, address() + size() - unused, unused, area_end() - unused);
            if (filler->address() != area_end()) {
                CHECK(filler->IsFiller());
                CHECK_EQ(filler->address() + filler->Size(), area_end());
            }
        }
        return unused;
    }

    void Page::CreateBlackArea(Address start, Address end)
    {
        DCHECK(heap()->incremental_marking()->black_allocation());
        DCHECK_EQ(Page::FromAddress(start), this);
        DCHECK_NE(start, end);
        DCHECK_EQ(Page::FromAddress(end - 1), this);
        IncrementalMarking::MarkingState* marking_state = heap()->incremental_marking()->marking_state();
        marking_state->bitmap(this)->SetRange(AddressToMarkbitIndex(start),
            AddressToMarkbitIndex(end));
        marking_state->IncrementLiveBytes(this, static_cast<intptr_t>(end - start));
    }

    void Page::DestroyBlackArea(Address start, Address end)
    {
        DCHECK(heap()->incremental_marking()->black_allocation());
        DCHECK_EQ(Page::FromAddress(start), this);
        DCHECK_NE(start, end);
        DCHECK_EQ(Page::FromAddress(end - 1), this);
        IncrementalMarking::MarkingState* marking_state = heap()->incremental_marking()->marking_state();
        marking_state->bitmap(this)->ClearRange(AddressToMarkbitIndex(start),
            AddressToMarkbitIndex(end));
        marking_state->IncrementLiveBytes(this, -static_cast<intptr_t>(end - start));
    }

    void MemoryAllocator::PartialFreeMemory(MemoryChunk* chunk, Address start_free,
        size_t bytes_to_free,
        Address new_area_end)
    {
        VirtualMemory* reservation = chunk->reserved_memory();
        DCHECK(reservation->IsReserved());
        chunk->size_ -= bytes_to_free;
        chunk->area_end_ = new_area_end;
        if (chunk->IsFlagSet(MemoryChunk::IS_EXECUTABLE)) {
            // Add guard page at the end.
            size_t page_size = GetCommitPageSize();
            DCHECK_EQ(0, chunk->area_end_ % static_cast<Address>(page_size));
            DCHECK_EQ(chunk->address() + chunk->size(),
                chunk->area_end() + MemoryChunkLayout::CodePageGuardSize());
            reservation->SetPermissions(chunk->area_end_, page_size,
                PageAllocator::kNoAccess);
        }
        // On e.g. Windows, a reservation may be larger than a page and releasing
        // partially starting at |start_free| will also release the potentially
        // unused part behind the current page.
        const size_t released_bytes = reservation->Release(start_free);
        DCHECK_GE(size_, released_bytes);
        size_ -= released_bytes;
        isolate_->counters()->memory_allocated()->Decrement(
            static_cast<int>(released_bytes));
    }

    void MemoryAllocator::PreFreeMemory(MemoryChunk* chunk)
    {
        DCHECK(!chunk->IsFlagSet(MemoryChunk::PRE_FREED));
        LOG(isolate_, DeleteEvent("MemoryChunk", chunk));

        isolate_->heap()->RememberUnmappedPage(reinterpret_cast<Address>(chunk),
            chunk->IsEvacuationCandidate());

        VirtualMemory* reservation = chunk->reserved_memory();
        const size_t size = reservation->IsReserved() ? reservation->size() : chunk->size();
        DCHECK_GE(size_, static_cast<size_t>(size));
        size_ -= size;
        isolate_->counters()->memory_allocated()->Decrement(static_cast<int>(size));
        if (chunk->executable() == EXECUTABLE) {
            DCHECK_GE(size_executable_, size);
            size_executable_ -= size;
        }

        chunk->SetFlag(MemoryChunk::PRE_FREED);

        if (chunk->executable())
            UnregisterExecutableMemoryChunk(chunk);
    }

    void MemoryAllocator::PerformFreeMemory(MemoryChunk* chunk)
    {
        DCHECK(chunk->IsFlagSet(MemoryChunk::PRE_FREED));
        chunk->ReleaseAllocatedMemory();

        VirtualMemory* reservation = chunk->reserved_memory();
        if (chunk->IsFlagSet(MemoryChunk::POOLED)) {
            UncommitMemory(reservation);
        } else {
            if (reservation->IsReserved()) {
                reservation->Free();
            } else {
                // Only read-only pages can have non-initialized reservation object.
                DCHECK_EQ(RO_SPACE, chunk->owner()->identity());
                FreeMemory(page_allocator(chunk->executable()), chunk->address(),
                    chunk->size());
            }
        }
    }

    template <MemoryAllocator::FreeMode mode>
    void MemoryAllocator::Free(MemoryChunk* chunk)
    {
        switch (mode) {
        case kFull:
            PreFreeMemory(chunk);
            PerformFreeMemory(chunk);
            break;
        case kAlreadyPooled:
            // Pooled pages cannot be touched anymore as their memory is uncommitted.
            // Pooled pages are not-executable.
            FreeMemory(data_page_allocator(), chunk->address(),
                static_cast<size_t>(MemoryChunk::kPageSize));
            break;
        case kPooledAndQueue:
            DCHECK_EQ(chunk->size(), static_cast<size_t>(MemoryChunk::kPageSize));
            DCHECK_EQ(chunk->executable(), NOT_EXECUTABLE);
            chunk->SetFlag(MemoryChunk::POOLED);
            V8_FALLTHROUGH;
        case kPreFreeAndQueue:
            PreFreeMemory(chunk);
            // The chunks added to this queue will be freed by a concurrent thread.
            unmapper()->AddMemoryChunkSafe(chunk);
            break;
        }
    }

    template EXPORT_TEMPLATE_DEFINE(V8_EXPORT_PRIVATE) void MemoryAllocator::Free<
        MemoryAllocator::kFull>(MemoryChunk* chunk);

    template EXPORT_TEMPLATE_DEFINE(V8_EXPORT_PRIVATE) void MemoryAllocator::Free<
        MemoryAllocator::kAlreadyPooled>(MemoryChunk* chunk);

    template EXPORT_TEMPLATE_DEFINE(V8_EXPORT_PRIVATE) void MemoryAllocator::Free<
        MemoryAllocator::kPreFreeAndQueue>(MemoryChunk* chunk);

    template EXPORT_TEMPLATE_DEFINE(V8_EXPORT_PRIVATE) void MemoryAllocator::Free<
        MemoryAllocator::kPooledAndQueue>(MemoryChunk* chunk);

    template <MemoryAllocator::AllocationMode alloc_mode, typename SpaceType>
    Page* MemoryAllocator::AllocatePage(size_t size, SpaceType* owner,
        Executability executable)
    {
        MemoryChunk* chunk = nullptr;
        if (alloc_mode == kPooled) {
            DCHECK_EQ(size, static_cast<size_t>(MemoryChunkLayout::AllocatableMemoryInMemoryChunk(owner->identity())));
            DCHECK_EQ(executable, NOT_EXECUTABLE);
            chunk = AllocatePagePooled(owner);
        }
        if (chunk == nullptr) {
            chunk = AllocateChunk(size, size, executable, owner);
        }
        if (chunk == nullptr)
            return nullptr;
        return owner->InitializePage(chunk, executable);
    }

    template EXPORT_TEMPLATE_DEFINE(V8_EXPORT_PRIVATE)
        Page* MemoryAllocator::AllocatePage<MemoryAllocator::kRegular, PagedSpace>(
            size_t size, PagedSpace* owner, Executability executable);
    template EXPORT_TEMPLATE_DEFINE(V8_EXPORT_PRIVATE)
        Page* MemoryAllocator::AllocatePage<MemoryAllocator::kRegular, SemiSpace>(
            size_t size, SemiSpace* owner, Executability executable);
    template EXPORT_TEMPLATE_DEFINE(V8_EXPORT_PRIVATE)
        Page* MemoryAllocator::AllocatePage<MemoryAllocator::kPooled, SemiSpace>(
            size_t size, SemiSpace* owner, Executability executable);

    LargePage* MemoryAllocator::AllocateLargePage(size_t size,
        LargeObjectSpace* owner,
        Executability executable)
    {
        MemoryChunk* chunk = AllocateChunk(size, size, executable, owner);
        if (chunk == nullptr)
            return nullptr;
        return LargePage::Initialize(isolate_->heap(), chunk, executable);
    }

    template <typename SpaceType>
    MemoryChunk* MemoryAllocator::AllocatePagePooled(SpaceType* owner)
    {
        MemoryChunk* chunk = unmapper()->TryGetPooledMemoryChunkSafe();
        if (chunk == nullptr)
            return nullptr;
        const int size = MemoryChunk::kPageSize;
        const Address start = reinterpret_cast<Address>(chunk);
        const Address area_start = start + MemoryChunkLayout::ObjectStartOffsetInMemoryChunk(owner->identity());
        const Address area_end = start + size;
        // Pooled pages are always regular data pages.
        DCHECK_NE(CODE_SPACE, owner->identity());
        VirtualMemory reservation(data_page_allocator(), start, size);
        if (!CommitMemory(&reservation))
            return nullptr;
        if (Heap::ShouldZapGarbage()) {
            ZapBlock(start, size, kZapValue);
        }
        MemoryChunk::Initialize(isolate_->heap(), start, size, area_start, area_end,
            NOT_EXECUTABLE, owner, std::move(reservation));
        size_ += size;
        return chunk;
    }

    void MemoryAllocator::ZapBlock(Address start, size_t size,
        uintptr_t zap_value)
    {
        DCHECK(IsAligned(start, kTaggedSize));
        DCHECK(IsAligned(size, kTaggedSize));
        MemsetTagged(ObjectSlot(start), Object(static_cast<Address>(zap_value)),
            size >> kTaggedSizeLog2);
    }

    intptr_t MemoryAllocator::GetCommitPageSize()
    {
        if (FLAG_v8_os_page_size != 0) {
            DCHECK(base::bits::IsPowerOfTwo(FLAG_v8_os_page_size));
            return FLAG_v8_os_page_size * KB;
        } else {
            return CommitPageSize();
        }
    }

    base::AddressRegion MemoryAllocator::ComputeDiscardMemoryArea(Address addr,
        size_t size)
    {
        size_t page_size = MemoryAllocator::GetCommitPageSize();
        if (size < page_size + FreeSpace::kSize) {
            return base::AddressRegion(0, 0);
        }
        Address discardable_start = RoundUp(addr + FreeSpace::kSize, page_size);
        Address discardable_end = RoundDown(addr + size, page_size);
        if (discardable_start >= discardable_end)
            return base::AddressRegion(0, 0);
        return base::AddressRegion(discardable_start,
            discardable_end - discardable_start);
    }

    bool MemoryAllocator::CommitExecutableMemory(VirtualMemory* vm, Address start,
        size_t commit_size,
        size_t reserved_size)
    {
        const size_t page_size = GetCommitPageSize();
        // All addresses and sizes must be aligned to the commit page size.
        DCHECK(IsAligned(start, page_size));
        DCHECK_EQ(0, commit_size % page_size);
        DCHECK_EQ(0, reserved_size % page_size);
        const size_t guard_size = MemoryChunkLayout::CodePageGuardSize();
        const size_t pre_guard_offset = MemoryChunkLayout::CodePageGuardStartOffset();
        const size_t code_area_offset = MemoryChunkLayout::ObjectStartOffsetInCodePage();
        // reserved_size includes two guard regions, commit_size does not.
        DCHECK_LE(commit_size, reserved_size - 2 * guard_size);
        const Address pre_guard_page = start + pre_guard_offset;
        const Address code_area = start + code_area_offset;
        const Address post_guard_page = start + reserved_size - guard_size;
        // Commit the non-executable header, from start to pre-code guard page.
        if (vm->SetPermissions(start, pre_guard_offset, PageAllocator::kReadWrite)) {
            // Create the pre-code guard page, following the header.
            if (vm->SetPermissions(pre_guard_page, page_size,
                    PageAllocator::kNoAccess)) {
                // Commit the executable code body.
                if (vm->SetPermissions(code_area, commit_size - pre_guard_offset,
                        PageAllocator::kReadWrite)) {
                    // Create the post-code guard page.
                    if (vm->SetPermissions(post_guard_page, page_size,
                            PageAllocator::kNoAccess)) {
                        UpdateAllocatedSpaceLimits(start, code_area + commit_size);
                        return true;
                    }
                    vm->SetPermissions(code_area, commit_size, PageAllocator::kNoAccess);
                }
            }
            vm->SetPermissions(start, pre_guard_offset, PageAllocator::kNoAccess);
        }
        return false;
    }

    // -----------------------------------------------------------------------------
    // MemoryChunk implementation

    void MemoryChunk::ReleaseAllocatedMemory()
    {
        if (skip_list_ != nullptr) {
            delete skip_list_;
            skip_list_ = nullptr;
        }
        if (mutex_ != nullptr) {
            delete mutex_;
            mutex_ = nullptr;
        }
        if (page_protection_change_mutex_ != nullptr) {
            delete page_protection_change_mutex_;
            page_protection_change_mutex_ = nullptr;
        }
        ReleaseSlotSet<OLD_TO_NEW>();
        ReleaseSlotSet<OLD_TO_OLD>();
        ReleaseTypedSlotSet<OLD_TO_NEW>();
        ReleaseTypedSlotSet<OLD_TO_OLD>();
        ReleaseInvalidatedSlots();
        if (local_tracker_ != nullptr)
            ReleaseLocalTracker();
        if (young_generation_bitmap_ != nullptr)
            ReleaseYoungGenerationBitmap();
        if (marking_bitmap_ != nullptr)
            ReleaseMarkingBitmap();

        if (!IsLargePage()) {
            Page* page = static_cast<Page*>(this);
            page->ReleaseFreeListCategories();
        }
    }

    static SlotSet* AllocateAndInitializeSlotSet(size_t size, Address page_start)
    {
        size_t pages = (size + Page::kPageSize - 1) / Page::kPageSize;
        DCHECK_LT(0, pages);
        SlotSet* slot_set = new SlotSet[pages];
        for (size_t i = 0; i < pages; i++) {
            slot_set[i].SetPageStart(page_start + i * Page::kPageSize);
        }
        return slot_set;
    }

    template V8_EXPORT_PRIVATE SlotSet* MemoryChunk::AllocateSlotSet<OLD_TO_NEW>();
    template V8_EXPORT_PRIVATE SlotSet* MemoryChunk::AllocateSlotSet<OLD_TO_OLD>();

    template <RememberedSetType type>
    SlotSet* MemoryChunk::AllocateSlotSet()
    {
        SlotSet* slot_set = AllocateAndInitializeSlotSet(size_, address());
        SlotSet* old_slot_set = base::AsAtomicPointer::Release_CompareAndSwap(
            &slot_set_[type], nullptr, slot_set);
        if (old_slot_set != nullptr) {
            delete[] slot_set;
            slot_set = old_slot_set;
        }
        DCHECK(slot_set);
        return slot_set;
    }

    template void MemoryChunk::ReleaseSlotSet<OLD_TO_NEW>();
    template void MemoryChunk::ReleaseSlotSet<OLD_TO_OLD>();

    template <RememberedSetType type>
    void MemoryChunk::ReleaseSlotSet()
    {
        SlotSet* slot_set = slot_set_[type];
        if (slot_set) {
            slot_set_[type] = nullptr;
            delete[] slot_set;
        }
    }

    template TypedSlotSet* MemoryChunk::AllocateTypedSlotSet<OLD_TO_NEW>();
    template TypedSlotSet* MemoryChunk::AllocateTypedSlotSet<OLD_TO_OLD>();

    template <RememberedSetType type>
    TypedSlotSet* MemoryChunk::AllocateTypedSlotSet()
    {
        TypedSlotSet* typed_slot_set = new TypedSlotSet(address());
        TypedSlotSet* old_value = base::AsAtomicPointer::Release_CompareAndSwap(
            &typed_slot_set_[type], nullptr, typed_slot_set);
        if (old_value != nullptr) {
            delete typed_slot_set;
            typed_slot_set = old_value;
        }
        DCHECK(typed_slot_set);
        return typed_slot_set;
    }

    template void MemoryChunk::ReleaseTypedSlotSet<OLD_TO_NEW>();
    template void MemoryChunk::ReleaseTypedSlotSet<OLD_TO_OLD>();

    template <RememberedSetType type>
    void MemoryChunk::ReleaseTypedSlotSet()
    {
        TypedSlotSet* typed_slot_set = typed_slot_set_[type];
        if (typed_slot_set) {
            typed_slot_set_[type] = nullptr;
            delete typed_slot_set;
        }
    }

    InvalidatedSlots* MemoryChunk::AllocateInvalidatedSlots()
    {
        DCHECK_NULL(invalidated_slots_);
        invalidated_slots_ = new InvalidatedSlots();
        return invalidated_slots_;
    }

    void MemoryChunk::ReleaseInvalidatedSlots()
    {
        if (invalidated_slots_) {
            delete invalidated_slots_;
            invalidated_slots_ = nullptr;
        }
    }

    void MemoryChunk::RegisterObjectWithInvalidatedSlots(HeapObject object,
        int size)
    {
        if (!ShouldSkipEvacuationSlotRecording()) {
            if (invalidated_slots() == nullptr) {
                AllocateInvalidatedSlots();
            }
            int old_size = (*invalidated_slots())[object];
            (*invalidated_slots())[object] = std::max(old_size, size);
        }
    }

    bool MemoryChunk::RegisteredObjectWithInvalidatedSlots(HeapObject object)
    {
        if (ShouldSkipEvacuationSlotRecording()) {
            // Invalidated slots do not matter if we are not recording slots.
            return true;
        }
        if (invalidated_slots() == nullptr) {
            return false;
        }
        return invalidated_slots()->find(object) != invalidated_slots()->end();
    }

    void MemoryChunk::MoveObjectWithInvalidatedSlots(HeapObject old_start,
        HeapObject new_start)
    {
        DCHECK_LT(old_start, new_start);
        DCHECK_EQ(MemoryChunk::FromHeapObject(old_start),
            MemoryChunk::FromHeapObject(new_start));
        if (!ShouldSkipEvacuationSlotRecording() && invalidated_slots()) {
            auto it = invalidated_slots()->find(old_start);
            if (it != invalidated_slots()->end()) {
                int old_size = it->second;
                int delta = static_cast<int>(new_start->address() - old_start->address());
                invalidated_slots()->erase(it);
                (*invalidated_slots())[new_start] = old_size - delta;
            }
        }
    }

    void MemoryChunk::ReleaseLocalTracker()
    {
        DCHECK_NOT_NULL(local_tracker_);
        delete local_tracker_;
        local_tracker_ = nullptr;
    }

    void MemoryChunk::AllocateYoungGenerationBitmap()
    {
        DCHECK_NULL(young_generation_bitmap_);
        young_generation_bitmap_ = static_cast<Bitmap*>(calloc(1, Bitmap::kSize));
    }

    void MemoryChunk::ReleaseYoungGenerationBitmap()
    {
        DCHECK_NOT_NULL(young_generation_bitmap_);
        free(young_generation_bitmap_);
        young_generation_bitmap_ = nullptr;
    }

    void MemoryChunk::AllocateMarkingBitmap()
    {
        DCHECK_NULL(marking_bitmap_);
        marking_bitmap_ = static_cast<Bitmap*>(calloc(1, Bitmap::kSize));
    }

    void MemoryChunk::ReleaseMarkingBitmap()
    {
        DCHECK_NOT_NULL(marking_bitmap_);
        free(marking_bitmap_);
        marking_bitmap_ = nullptr;
    }

    // -----------------------------------------------------------------------------
    // PagedSpace implementation

    void Space::CheckOffsetsAreConsistent() const
    {
        static_assert(Space::kIdOffset == heap_internals::Space::kIdOffset,
            "ID offset inconsistent");
        DCHECK_EQ(Space::kIdOffset, OFFSET_OF(Space, id_));
    }

    void Space::AddAllocationObserver(AllocationObserver* observer)
    {
        allocation_observers_->push_back(observer);
        StartNextInlineAllocationStep();
    }

    void Space::RemoveAllocationObserver(AllocationObserver* observer)
    {
        auto it = std::find(allocation_observers_->begin(),
            allocation_observers_->end(), observer);
        DCHECK(allocation_observers_->end() != it);
        allocation_observers_->erase(it);
        StartNextInlineAllocationStep();
    }

    void Space::PauseAllocationObservers() { allocation_observers_paused_ = true; }

    void Space::ResumeAllocationObservers()
    {
        allocation_observers_paused_ = false;
    }

    void Space::AllocationStep(int bytes_since_last, Address soon_object,
        int size)
    {
        if (!AllocationObserversActive()) {
            return;
        }

        DCHECK(!heap()->allocation_step_in_progress());
        heap()->set_allocation_step_in_progress(true);
        heap()->CreateFillerObjectAt(soon_object, size, ClearRecordedSlots::kNo);
        for (AllocationObserver* observer : *allocation_observers_) {
            observer->AllocationStep(bytes_since_last, soon_object, size);
        }
        heap()->set_allocation_step_in_progress(false);
    }

    intptr_t Space::GetNextInlineAllocationStepSize()
    {
        intptr_t next_step = 0;
        for (AllocationObserver* observer : *allocation_observers_) {
            next_step = next_step ? Min(next_step, observer->bytes_to_next_step())
                                  : observer->bytes_to_next_step();
        }
        DCHECK(allocation_observers_->size() == 0 || next_step > 0);
        return next_step;
    }

    PagedSpace::PagedSpace(Heap* heap, AllocationSpace space,
        Executability executable)
        : SpaceWithLinearArea(heap, space)
        , executable_(executable)
    {
        area_size_ = MemoryChunkLayout::AllocatableMemoryInMemoryChunk(space);
        accounting_stats_.Clear();
    }

    void PagedSpace::TearDown()
    {
        while (!memory_chunk_list_.Empty()) {
            MemoryChunk* chunk = memory_chunk_list_.front();
            memory_chunk_list_.Remove(chunk);
            heap()->memory_allocator()->Free<MemoryAllocator::kFull>(chunk);
        }
        accounting_stats_.Clear();
    }

    void PagedSpace::RefillFreeList()
    {
        // Any PagedSpace might invoke RefillFreeList. We filter all but our old
        // generation spaces out.
        if (identity() != OLD_SPACE && identity() != CODE_SPACE && identity() != MAP_SPACE && identity() != RO_SPACE) {
            return;
        }
        MarkCompactCollector* collector = heap()->mark_compact_collector();
        size_t added = 0;
        {
            Page* p = nullptr;
            while ((p = collector->sweeper()->GetSweptPageSafe(this)) != nullptr) {
                // We regularly sweep NEVER_ALLOCATE_ON_PAGE pages. We drop the freelist
                // entries here to make them unavailable for allocations.
                if (p->IsFlagSet(Page::NEVER_ALLOCATE_ON_PAGE)) {
                    p->ForAllFreeListCategories(
                        [](FreeListCategory* category) { category->Reset(); });
                }
                // Only during compaction pages can actually change ownership. This is
                // safe because there exists no other competing action on the page links
                // during compaction.
                if (is_local()) {
                    DCHECK_NE(this, p->owner());
                    PagedSpace* owner = reinterpret_cast<PagedSpace*>(p->owner());
                    base::MutexGuard guard(owner->mutex());
                    owner->RefineAllocatedBytesAfterSweeping(p);
                    owner->RemovePage(p);
                    added += AddPage(p);
                } else {
                    base::MutexGuard guard(mutex());
                    DCHECK_EQ(this, p->owner());
                    RefineAllocatedBytesAfterSweeping(p);
                    added += RelinkFreeListCategories(p);
                }
                added += p->wasted_memory();
                if (is_local() && (added > kCompactionMemoryWanted))
                    break;
            }
        }
    }

    void PagedSpace::MergeCompactionSpace(CompactionSpace* other)
    {
        base::MutexGuard guard(mutex());

        DCHECK(identity() == other->identity());
        // Unmerged fields:
        //   area_size_
        other->FreeLinearAllocationArea();

        // The linear allocation area of {other} should be destroyed now.
        DCHECK_EQ(kNullAddress, other->top());
        DCHECK_EQ(kNullAddress, other->limit());

        // Move over pages.
        for (auto it = other->begin(); it != other->end();) {
            Page* p = *(it++);
            // Relinking requires the category to be unlinked.
            other->RemovePage(p);
            AddPage(p);
            DCHECK_IMPLIES(
                !p->IsFlagSet(Page::NEVER_ALLOCATE_ON_PAGE),
                p->AvailableInFreeList() == p->AvailableInFreeListFromAllocatedBytes());
        }
        DCHECK_EQ(0u, other->Size());
        DCHECK_EQ(0u, other->Capacity());
    }

    size_t PagedSpace::CommittedPhysicalMemory()
    {
        if (!base::OS::HasLazyCommits())
            return CommittedMemory();
        MemoryChunk::UpdateHighWaterMark(allocation_info_.top());
        size_t size = 0;
        for (Page* page : *this) {
            size += page->CommittedPhysicalMemory();
        }
        return size;
    }

    bool PagedSpace::ContainsSlow(Address addr)
    {
        Page* p = Page::FromAddress(addr);
        for (Page* page : *this) {
            if (page == p)
                return true;
        }
        return false;
    }

    void PagedSpace::RefineAllocatedBytesAfterSweeping(Page* page)
    {
        CHECK(page->SweepingDone());
        auto marking_state = heap()->incremental_marking()->non_atomic_marking_state();
        // The live_byte on the page was accounted in the space allocated
        // bytes counter. After sweeping allocated_bytes() contains the
        // accurate live byte count on the page.
        size_t old_counter = marking_state->live_bytes(page);
        size_t new_counter = page->allocated_bytes();
        DCHECK_GE(old_counter, new_counter);
        if (old_counter > new_counter) {
            DecreaseAllocatedBytes(old_counter - new_counter, page);
            // Give the heap a chance to adjust counters in response to the
            // more precise and smaller old generation size.
            heap()->NotifyRefinedOldGenerationSize(old_counter - new_counter);
        }
        marking_state->SetLiveBytes(page, 0);
    }

    Page* PagedSpace::RemovePageSafe(int size_in_bytes)
    {
        base::MutexGuard guard(mutex());
        // Check for pages that still contain free list entries. Bail out for smaller
        // categories.
        const int minimum_category = static_cast<int>(FreeList::SelectFreeListCategoryType(size_in_bytes));
        Page* page = free_list()->GetPageForCategoryType(kHuge);
        if (!page && static_cast<int>(kLarge) >= minimum_category)
            page = free_list()->GetPageForCategoryType(kLarge);
        if (!page && static_cast<int>(kMedium) >= minimum_category)
            page = free_list()->GetPageForCategoryType(kMedium);
        if (!page && static_cast<int>(kSmall) >= minimum_category)
            page = free_list()->GetPageForCategoryType(kSmall);
        if (!page && static_cast<int>(kTiny) >= minimum_category)
            page = free_list()->GetPageForCategoryType(kTiny);
        if (!page && static_cast<int>(kTiniest) >= minimum_category)
            page = free_list()->GetPageForCategoryType(kTiniest);
        if (!page)
            return nullptr;
        RemovePage(page);
        return page;
    }

    size_t PagedSpace::AddPage(Page* page)
    {
        CHECK(page->SweepingDone());
        page->set_owner(this);
        memory_chunk_list_.PushBack(page);
        AccountCommitted(page->size());
        IncreaseCapacity(page->area_size());
        IncreaseAllocatedBytes(page->allocated_bytes(), page);
        for (size_t i = 0; i < ExternalBackingStoreType::kNumTypes; i++) {
            ExternalBackingStoreType t = static_cast<ExternalBackingStoreType>(i);
            IncrementExternalBackingStoreBytes(t, page->ExternalBackingStoreBytes(t));
        }
        return RelinkFreeListCategories(page);
    }

    void PagedSpace::RemovePage(Page* page)
    {
        CHECK(page->SweepingDone());
        memory_chunk_list_.Remove(page);
        UnlinkFreeListCategories(page);
        DecreaseAllocatedBytes(page->allocated_bytes(), page);
        DecreaseCapacity(page->area_size());
        AccountUncommitted(page->size());
        for (size_t i = 0; i < ExternalBackingStoreType::kNumTypes; i++) {
            ExternalBackingStoreType t = static_cast<ExternalBackingStoreType>(i);
            DecrementExternalBackingStoreBytes(t, page->ExternalBackingStoreBytes(t));
        }
    }

    size_t PagedSpace::ShrinkPageToHighWaterMark(Page* page)
    {
        size_t unused = page->ShrinkToHighWaterMark();
        accounting_stats_.DecreaseCapacity(static_cast<intptr_t>(unused));
        AccountUncommitted(unused);
        return unused;
    }

    void PagedSpace::ResetFreeList()
    {
        for (Page* page : *this) {
            free_list_.EvictFreeListItems(page);
        }
        DCHECK(free_list_.IsEmpty());
    }

    void PagedSpace::ShrinkImmortalImmovablePages()
    {
        DCHECK(!heap()->deserialization_complete());
        MemoryChunk::UpdateHighWaterMark(allocation_info_.top());
        FreeLinearAllocationArea();
        ResetFreeList();
        for (Page* page : *this) {
            DCHECK(page->IsFlagSet(Page::NEVER_EVACUATE));
            ShrinkPageToHighWaterMark(page);
        }
    }

    bool PagedSpace::Expand()
    {
        // Always lock against the main space as we can only adjust capacity and
        // pages concurrently for the main paged space.
        base::MutexGuard guard(heap()->paged_space(identity())->mutex());

        const int size = AreaSize();

        if (!heap()->CanExpandOldGeneration(size))
            return false;

        Page* page = heap()->memory_allocator()->AllocatePage(size, this, executable());
        if (page == nullptr)
            return false;
        // Pages created during bootstrapping may contain immortal immovable objects.
        if (!heap()->deserialization_complete())
            page->MarkNeverEvacuate();
        AddPage(page);
        Free(page->area_start(), page->area_size(),
            SpaceAccountingMode::kSpaceAccounted);
        heap()->NotifyOldGenerationExpansion();
        return true;
    }

    int PagedSpace::CountTotalPages()
    {
        int count = 0;
        for (Page* page : *this) {
            count++;
            USE(page);
        }
        return count;
    }

    void PagedSpace::ResetFreeListStatistics()
    {
        for (Page* page : *this) {
            page->ResetFreeListStatistics();
        }
    }

    void PagedSpace::SetLinearAllocationArea(Address top, Address limit)
    {
        SetTopAndLimit(top, limit);
        if (top != kNullAddress && top != limit && heap()->incremental_marking()->black_allocation()) {
            Page::FromAllocationAreaAddress(top)->CreateBlackArea(top, limit);
        }
    }

    void PagedSpace::DecreaseLimit(Address new_limit)
    {
        Address old_limit = limit();
        DCHECK_LE(top(), new_limit);
        DCHECK_GE(old_limit, new_limit);
        if (new_limit != old_limit) {
            SetTopAndLimit(top(), new_limit);
            Free(new_limit, old_limit - new_limit,
                SpaceAccountingMode::kSpaceAccounted);
            if (heap()->incremental_marking()->black_allocation()) {
                Page::FromAllocationAreaAddress(new_limit)->DestroyBlackArea(new_limit,
                    old_limit);
            }
        }
    }

    Address SpaceWithLinearArea::ComputeLimit(Address start, Address end,
        size_t min_size)
    {
        DCHECK_GE(end - start, min_size);

        if (heap()->inline_allocation_disabled()) {
            // Fit the requested area exactly.
            return start + min_size;
        } else if (SupportsInlineAllocation() && AllocationObserversActive()) {
            // Generated code may allocate inline from the linear allocation area for.
            // To make sure we can observe these allocations, we use a lower limit.
            size_t step = GetNextInlineAllocationStepSize();

            // TODO(ofrobots): there is subtle difference between old space and new
            // space here. Any way to avoid it? `step - 1` makes more sense as we would
            // like to sample the object that straddles the `start + step` boundary.
            // Rounding down further would introduce a small statistical error in
            // sampling. However, presently PagedSpace requires limit to be aligned.
            size_t rounded_step;
            if (identity() == NEW_SPACE) {
                DCHECK_GE(step, 1);
                rounded_step = step - 1;
            } else {
                rounded_step = RoundSizeDownToObjectAlignment(static_cast<int>(step));
            }
            return Min(static_cast<Address>(start + min_size + rounded_step), end);
        } else {
            // The entire node can be used as the linear allocation area.
            return end;
        }
    }

    void PagedSpace::MarkLinearAllocationAreaBlack()
    {
        DCHECK(heap()->incremental_marking()->black_allocation());
        Address current_top = top();
        Address current_limit = limit();
        if (current_top != kNullAddress && current_top != current_limit) {
            Page::FromAllocationAreaAddress(current_top)
                ->CreateBlackArea(current_top, current_limit);
        }
    }

    void PagedSpace::UnmarkLinearAllocationArea()
    {
        Address current_top = top();
        Address current_limit = limit();
        if (current_top != kNullAddress && current_top != current_limit) {
            Page::FromAllocationAreaAddress(current_top)
                ->DestroyBlackArea(current_top, current_limit);
        }
    }

    void PagedSpace::FreeLinearAllocationArea()
    {
        // Mark the old linear allocation area with a free space map so it can be
        // skipped when scanning the heap.
        Address current_top = top();
        Address current_limit = limit();
        if (current_top == kNullAddress) {
            DCHECK_EQ(kNullAddress, current_limit);
            return;
        }

        if (heap()->incremental_marking()->black_allocation()) {
            Page* page = Page::FromAllocationAreaAddress(current_top);

            // Clear the bits in the unused black area.
            if (current_top != current_limit) {
                IncrementalMarking::MarkingState* marking_state = heap()->incremental_marking()->marking_state();
                marking_state->bitmap(page)->ClearRange(
                    page->AddressToMarkbitIndex(current_top),
                    page->AddressToMarkbitIndex(current_limit));
                marking_state->IncrementLiveBytes(
                    page, -static_cast<int>(current_limit - current_top));
            }
        }

        InlineAllocationStep(current_top, kNullAddress, kNullAddress, 0);
        SetTopAndLimit(kNullAddress, kNullAddress);
        DCHECK_GE(current_limit, current_top);

        // The code page of the linear allocation area needs to be unprotected
        // because we are going to write a filler into that memory area below.
        if (identity() == CODE_SPACE) {
            heap()->UnprotectAndRegisterMemoryChunk(
                MemoryChunk::FromAddress(current_top));
        }
        Free(current_top, current_limit - current_top,
            SpaceAccountingMode::kSpaceAccounted);
    }

    void PagedSpace::ReleasePage(Page* page)
    {
        DCHECK_EQ(
            0, heap()->incremental_marking()->non_atomic_marking_state()->live_bytes(page));
        DCHECK_EQ(page->owner(), this);

        free_list_.EvictFreeListItems(page);
        DCHECK(!free_list_.ContainsPageFreeListItems(page));

        if (Page::FromAllocationAreaAddress(allocation_info_.top()) == page) {
            DCHECK(!top_on_previous_step_);
            allocation_info_.Reset(kNullAddress, kNullAddress);
        }

        AccountUncommitted(page->size());
        accounting_stats_.DecreaseCapacity(page->area_size());
        heap()->memory_allocator()->Free<MemoryAllocator::kPreFreeAndQueue>(page);
    }

    void PagedSpace::SetReadable()
    {
        DCHECK(identity() == CODE_SPACE);
        for (Page* page : *this) {
            CHECK(heap()->memory_allocator()->IsMemoryChunkExecutable(page));
            page->SetReadable();
        }
    }

    void PagedSpace::SetReadAndExecutable()
    {
        DCHECK(identity() == CODE_SPACE);
        for (Page* page : *this) {
            CHECK(heap()->memory_allocator()->IsMemoryChunkExecutable(page));
            page->SetReadAndExecutable();
        }
    }

    void PagedSpace::SetReadAndWritable()
    {
        DCHECK(identity() == CODE_SPACE);
        for (Page* page : *this) {
            CHECK(heap()->memory_allocator()->IsMemoryChunkExecutable(page));
            page->SetReadAndWritable();
        }
    }

    std::unique_ptr<ObjectIterator> PagedSpace::GetObjectIterator()
    {
        return std::unique_ptr<ObjectIterator>(new HeapObjectIterator(this));
    }

    bool PagedSpace::RefillLinearAllocationAreaFromFreeList(size_t size_in_bytes)
    {
        DCHECK(IsAligned(size_in_bytes, kTaggedSize));
        DCHECK_LE(top(), limit());
#ifdef DEBUG
        if (top() != limit()) {
            DCHECK_EQ(Page::FromAddress(top()), Page::FromAddress(limit() - 1));
        }
#endif
        // Don't free list allocate if there is linear space available.
        DCHECK_LT(static_cast<size_t>(limit() - top()), size_in_bytes);

        // Mark the old linear allocation area with a free space map so it can be
        // skipped when scanning the heap.  This also puts it back in the free list
        // if it is big enough.
        FreeLinearAllocationArea();

        if (!is_local()) {
            heap()->StartIncrementalMarkingIfAllocationLimitIsReached(
                heap()->GCFlagsForIncrementalMarking(),
                kGCCallbackScheduleIdleGarbageCollection);
        }

        size_t new_node_size = 0;
        FreeSpace new_node = free_list_.Allocate(size_in_bytes, &new_node_size);
        if (new_node.is_null())
            return false;

        DCHECK_GE(new_node_size, size_in_bytes);

        // The old-space-step might have finished sweeping and restarted marking.
        // Verify that it did not turn the page of the new node into an evacuation
        // candidate.
        DCHECK(!MarkCompactCollector::IsOnEvacuationCandidate(new_node));

        // Memory in the linear allocation area is counted as allocated.  We may free
        // a little of this again immediately - see below.
        Page* page = Page::FromHeapObject(new_node);
        IncreaseAllocatedBytes(new_node_size, page);

        Address start = new_node->address();
        Address end = new_node->address() + new_node_size;
        Address limit = ComputeLimit(start, end, size_in_bytes);
        DCHECK_LE(limit, end);
        DCHECK_LE(size_in_bytes, limit - start);
        if (limit != end) {
            if (identity() == CODE_SPACE) {
                heap()->UnprotectAndRegisterMemoryChunk(page);
            }
            Free(limit, end - limit, SpaceAccountingMode::kSpaceAccounted);
        }
        SetLinearAllocationArea(start, limit);

        return true;
    }

#ifdef DEBUG
    void PagedSpace::Print()
    {
    }
#endif

#ifdef VERIFY_HEAP
    void PagedSpace::Verify(Isolate* isolate, ObjectVisitor* visitor)
    {
        bool allocation_pointer_found_in_space = (allocation_info_.top() == allocation_info_.limit());
        size_t external_space_bytes[kNumTypes];
        size_t external_page_bytes[kNumTypes];

        for (int i = 0; i < kNumTypes; i++) {
            external_space_bytes[static_cast<ExternalBackingStoreType>(i)] = 0;
        }

        for (Page* page : *this) {
            CHECK(page->owner() == this);

            for (int i = 0; i < kNumTypes; i++) {
                external_page_bytes[static_cast<ExternalBackingStoreType>(i)] = 0;
            }

            if (page == Page::FromAllocationAreaAddress(allocation_info_.top())) {
                allocation_pointer_found_in_space = true;
            }
            CHECK(page->SweepingDone());
            HeapObjectIterator it(page);
            Address end_of_previous_object = page->area_start();
            Address top = page->area_end();

            for (HeapObject object = it.Next(); !object.is_null(); object = it.Next()) {
                CHECK(end_of_previous_object <= object->address());

                // The first word should be a map, and we expect all map pointers to
                // be in map space.
                Map map = object->map();
                CHECK(map->IsMap());
                CHECK(heap()->map_space()->Contains(map) || heap()->read_only_space()->Contains(map));

                // Perform space-specific object verification.
                VerifyObject(object);

                // The object itself should look OK.
                object->ObjectVerify(isolate);

                if (!FLAG_verify_heap_skip_remembered_set) {
                    heap()->VerifyRememberedSetFor(object);
                }

                // All the interior pointers should be contained in the heap.
                int size = object->Size();
                object->IterateBody(map, size, visitor);
                CHECK(object->address() + size <= top);
                end_of_previous_object = object->address() + size;

                if (object->IsExternalString()) {
                    ExternalString external_string = ExternalString::cast(object);
                    size_t size = external_string->ExternalPayloadSize();
                    external_page_bytes[ExternalBackingStoreType::kExternalString] += size;
                } else if (object->IsJSArrayBuffer()) {
                    JSArrayBuffer array_buffer = JSArrayBuffer::cast(object);
                    if (ArrayBufferTracker::IsTracked(array_buffer)) {
                        size_t size = array_buffer->byte_length();
                        external_page_bytes[ExternalBackingStoreType::kArrayBuffer] += size;
                    }
                }
            }
            for (int i = 0; i < kNumTypes; i++) {
                ExternalBackingStoreType t = static_cast<ExternalBackingStoreType>(i);
                CHECK_EQ(external_page_bytes[t], page->ExternalBackingStoreBytes(t));
                external_space_bytes[t] += external_page_bytes[t];
            }
        }
        for (int i = 0; i < kNumTypes; i++) {
            ExternalBackingStoreType t = static_cast<ExternalBackingStoreType>(i);
            CHECK_EQ(external_space_bytes[t], ExternalBackingStoreBytes(t));
        }
        CHECK(allocation_pointer_found_in_space);
#ifdef DEBUG
        VerifyCountersAfterSweeping();
#endif
    }

    void PagedSpace::VerifyLiveBytes()
    {
        IncrementalMarking::MarkingState* marking_state = heap()->incremental_marking()->marking_state();
        for (Page* page : *this) {
            CHECK(page->SweepingDone());
            HeapObjectIterator it(page);
            int black_size = 0;
            for (HeapObject object = it.Next(); !object.is_null(); object = it.Next()) {
                // All the interior pointers should be contained in the heap.
                if (marking_state->IsBlack(object)) {
                    black_size += object->Size();
                }
            }
            CHECK_LE(black_size, marking_state->live_bytes(page));
        }
    }
#endif // VERIFY_HEAP

#ifdef DEBUG
    void PagedSpace::VerifyCountersAfterSweeping()
    {
        size_t total_capacity = 0;
        size_t total_allocated = 0;
        for (Page* page : *this) {
            DCHECK(page->SweepingDone());
            total_capacity += page->area_size();
            HeapObjectIterator it(page);
            size_t real_allocated = 0;
            for (HeapObject object = it.Next(); !object.is_null(); object = it.Next()) {
                if (!object->IsFiller()) {
                    real_allocated += object->Size();
                }
            }
            total_allocated += page->allocated_bytes();
            // The real size can be smaller than the accounted size if array trimming,
            // object slack tracking happened after sweeping.
            DCHECK_LE(real_allocated, accounting_stats_.AllocatedOnPage(page));
            DCHECK_EQ(page->allocated_bytes(), accounting_stats_.AllocatedOnPage(page));
        }
        DCHECK_EQ(total_capacity, accounting_stats_.Capacity());
        DCHECK_EQ(total_allocated, accounting_stats_.Size());
    }

    void PagedSpace::VerifyCountersBeforeConcurrentSweeping()
    {
        // We need to refine the counters on pages that are already swept and have
        // not been moved over to the actual space. Otherwise, the AccountingStats
        // are just an over approximation.
        RefillFreeList();

        size_t total_capacity = 0;
        size_t total_allocated = 0;
        auto marking_state = heap()->incremental_marking()->non_atomic_marking_state();
        for (Page* page : *this) {
            size_t page_allocated = page->SweepingDone()
                ? page->allocated_bytes()
                : static_cast<size_t>(marking_state->live_bytes(page));
            total_capacity += page->area_size();
            total_allocated += page_allocated;
            DCHECK_EQ(page_allocated, accounting_stats_.AllocatedOnPage(page));
        }
        DCHECK_EQ(total_capacity, accounting_stats_.Capacity());
        DCHECK_EQ(total_allocated, accounting_stats_.Size());
    }
#endif

    // -----------------------------------------------------------------------------
    // NewSpace implementation

    NewSpace::NewSpace(Heap* heap, v8::PageAllocator* page_allocator,
        size_t initial_semispace_capacity,
        size_t max_semispace_capacity)
        : SpaceWithLinearArea(heap, NEW_SPACE)
        , to_space_(heap, kToSpace)
        , from_space_(heap, kFromSpace)
    {
        DCHECK(initial_semispace_capacity <= max_semispace_capacity);
        DCHECK(
            base::bits::IsPowerOfTwo(static_cast<uint32_t>(max_semispace_capacity)));

        to_space_.SetUp(initial_semispace_capacity, max_semispace_capacity);
        from_space_.SetUp(initial_semispace_capacity, max_semispace_capacity);
        if (!to_space_.Commit()) {
            V8::FatalProcessOutOfMemory(heap->isolate(), "New space setup");
        }
        DCHECK(!from_space_.is_committed()); // No need to use memory yet.
        ResetLinearAllocationArea();
    }

    void NewSpace::TearDown()
    {
        allocation_info_.Reset(kNullAddress, kNullAddress);

        to_space_.TearDown();
        from_space_.TearDown();
    }

    void NewSpace::Flip() { SemiSpace::Swap(&from_space_, &to_space_); }

    void NewSpace::Grow()
    {
        // Double the semispace size but only up to maximum capacity.
        DCHECK(TotalCapacity() < MaximumCapacity());
        size_t new_capacity = Min(MaximumCapacity(),
            static_cast<size_t>(FLAG_semi_space_growth_factor) * TotalCapacity());
        if (to_space_.GrowTo(new_capacity)) {
            // Only grow from space if we managed to grow to-space.
            if (!from_space_.GrowTo(new_capacity)) {
                // If we managed to grow to-space but couldn't grow from-space,
                // attempt to shrink to-space.
                if (!to_space_.ShrinkTo(from_space_.current_capacity())) {
                    // We are in an inconsistent state because we could not
                    // commit/uncommit memory from new space.
                    FATAL("inconsistent state");
                }
            }
        }
        DCHECK_SEMISPACE_ALLOCATION_INFO(allocation_info_, to_space_);
    }

    void NewSpace::Shrink()
    {
        size_t new_capacity = Max(InitialTotalCapacity(), 2 * Size());
        size_t rounded_new_capacity = ::RoundUp(new_capacity, Page::kPageSize);
        if (rounded_new_capacity < TotalCapacity() && to_space_.ShrinkTo(rounded_new_capacity)) {
            // Only shrink from-space if we managed to shrink to-space.
            from_space_.Reset();
            if (!from_space_.ShrinkTo(rounded_new_capacity)) {
                // If we managed to shrink to-space but couldn't shrink from
                // space, attempt to grow to-space again.
                if (!to_space_.GrowTo(from_space_.current_capacity())) {
                    // We are in an inconsistent state because we could not
                    // commit/uncommit memory from new space.
                    FATAL("inconsistent state");
                }
            }
        }
        DCHECK_SEMISPACE_ALLOCATION_INFO(allocation_info_, to_space_);
    }

    bool NewSpace::Rebalance()
    {
        // Order here is important to make use of the page pool.
        return to_space_.EnsureCurrentCapacity() && from_space_.EnsureCurrentCapacity();
    }

    bool SemiSpace::EnsureCurrentCapacity()
    {
        if (is_committed()) {
            const int expected_pages = static_cast<int>(current_capacity_ / Page::kPageSize);
            MemoryChunk* current_page = first_page();
            int actual_pages = 0;

            // First iterate through the pages list until expected pages if so many
            // pages exist.
            while (current_page != nullptr && actual_pages < expected_pages) {
                actual_pages++;
                current_page = current_page->list_node().next();
            }

            // Free all overallocated pages which are behind current_page.
            while (current_page) {
                MemoryChunk* next_current = current_page->list_node().next();
                memory_chunk_list_.Remove(current_page);
                // Clear new space flags to avoid this page being treated as a new
                // space page that is potentially being swept.
                current_page->SetFlags(0, Page::kIsInYoungGenerationMask);
                heap()->memory_allocator()->Free<MemoryAllocator::kPooledAndQueue>(
                    current_page);
                current_page = next_current;
            }

            // Add more pages if we have less than expected_pages.
            IncrementalMarking::NonAtomicMarkingState* marking_state = heap()->incremental_marking()->non_atomic_marking_state();
            while (actual_pages < expected_pages) {
                actual_pages++;
                current_page = heap()->memory_allocator()->AllocatePage<MemoryAllocator::kPooled>(
                    MemoryChunkLayout::AllocatableMemoryInDataPage(), this,
                    NOT_EXECUTABLE);
                if (current_page == nullptr)
                    return false;
                DCHECK_NOT_NULL(current_page);
                memory_chunk_list_.PushBack(current_page);
                marking_state->ClearLiveness(current_page);
                current_page->SetFlags(first_page()->GetFlags(),
                    static_cast<uintptr_t>(Page::kCopyAllFlags));
                heap()->CreateFillerObjectAt(current_page->area_start(),
                    static_cast<int>(current_page->area_size()),
                    ClearRecordedSlots::kNo);
            }
        }
        return true;
    }

    LinearAllocationArea LocalAllocationBuffer::Close()
    {
        if (IsValid()) {
            heap_->CreateFillerObjectAt(
                allocation_info_.top(),
                static_cast<int>(allocation_info_.limit() - allocation_info_.top()),
                ClearRecordedSlots::kNo);
            const LinearAllocationArea old_info = allocation_info_;
            allocation_info_ = LinearAllocationArea(kNullAddress, kNullAddress);
            return old_info;
        }
        return LinearAllocationArea(kNullAddress, kNullAddress);
    }

    LocalAllocationBuffer::LocalAllocationBuffer(
        Heap* heap, LinearAllocationArea allocation_info) V8_NOEXCEPT
        : heap_(heap),
          allocation_info_(allocation_info)
    {
        if (IsValid()) {
            heap_->CreateFillerObjectAt(
                allocation_info_.top(),
                static_cast<int>(allocation_info_.limit() - allocation_info_.top()),
                ClearRecordedSlots::kNo);
        }
    }

    LocalAllocationBuffer::LocalAllocationBuffer(const LocalAllocationBuffer& other)
        V8_NOEXCEPT
    {
        *this = other;
    }

    LocalAllocationBuffer& LocalAllocationBuffer::operator=(
        const LocalAllocationBuffer& other) V8_NOEXCEPT
    {
        Close();
        heap_ = other.heap_;
        allocation_info_ = other.allocation_info_;

        // This is needed since we (a) cannot yet use move-semantics, and (b) want
        // to make the use of the class easy by it as value and (c) implicitly call
        // {Close} upon copy.
        const_cast<LocalAllocationBuffer&>(other).allocation_info_.Reset(
            kNullAddress, kNullAddress);
        return *this;
    }

    void NewSpace::UpdateLinearAllocationArea()
    {
        // Make sure there is no unaccounted allocations.
        DCHECK(!AllocationObserversActive() || top_on_previous_step_ == top());

        Address new_top = to_space_.page_low();
        MemoryChunk::UpdateHighWaterMark(allocation_info_.top());
        allocation_info_.Reset(new_top, to_space_.page_high());
        // The order of the following two stores is important.
        // See the corresponding loads in ConcurrentMarking::Run.
        original_limit_.store(limit(), std::memory_order_relaxed);
        original_top_.store(top(), std::memory_order_release);
        StartNextInlineAllocationStep();
        DCHECK_SEMISPACE_ALLOCATION_INFO(allocation_info_, to_space_);
    }

    void NewSpace::ResetLinearAllocationArea()
    {
        // Do a step to account for memory allocated so far before resetting.
        InlineAllocationStep(top(), top(), kNullAddress, 0);
        to_space_.Reset();
        UpdateLinearAllocationArea();
        // Clear all mark-bits in the to-space.
        IncrementalMarking::NonAtomicMarkingState* marking_state = heap()->incremental_marking()->non_atomic_marking_state();
        for (Page* p : to_space_) {
            marking_state->ClearLiveness(p);
            // Concurrent marking may have local live bytes for this page.
            heap()->concurrent_marking()->ClearMemoryChunkData(p);
        }
    }

    void NewSpace::UpdateInlineAllocationLimit(size_t min_size)
    {
        Address new_limit = ComputeLimit(top(), to_space_.page_high(), min_size);
        allocation_info_.set_limit(new_limit);
        DCHECK_SEMISPACE_ALLOCATION_INFO(allocation_info_, to_space_);
    }

    void PagedSpace::UpdateInlineAllocationLimit(size_t min_size)
    {
        Address new_limit = ComputeLimit(top(), limit(), min_size);
        DCHECK_LE(new_limit, limit());
        DecreaseLimit(new_limit);
    }

    bool NewSpace::AddFreshPage()
    {
        Address top = allocation_info_.top();
        DCHECK(!OldSpace::IsAtPageStart(top));

        // Do a step to account for memory allocated on previous page.
        InlineAllocationStep(top, top, kNullAddress, 0);

        if (!to_space_.AdvancePage()) {
            // No more pages left to advance.
            return false;
        }

        // Clear remainder of current page.
        Address limit = Page::FromAllocationAreaAddress(top)->area_end();
        int remaining_in_page = static_cast<int>(limit - top);
        heap()->CreateFillerObjectAt(top, remaining_in_page, ClearRecordedSlots::kNo);
        UpdateLinearAllocationArea();

        return true;
    }

    bool NewSpace::AddFreshPageSynchronized()
    {
        base::MutexGuard guard(&mutex_);
        return AddFreshPage();
    }

    bool NewSpace::EnsureAllocation(int size_in_bytes,
        AllocationAlignment alignment)
    {
        Address old_top = allocation_info_.top();
        Address high = to_space_.page_high();
        int filler_size = Heap::GetFillToAlign(old_top, alignment);
        int aligned_size_in_bytes = size_in_bytes + filler_size;

        if (old_top + aligned_size_in_bytes > high) {
            // Not enough room in the page, try to allocate a new one.
            if (!AddFreshPage()) {
                return false;
            }

            old_top = allocation_info_.top();
            high = to_space_.page_high();
            filler_size = Heap::GetFillToAlign(old_top, alignment);
        }

        DCHECK(old_top + aligned_size_in_bytes <= high);

        if (allocation_info_.limit() < high) {
            // Either the limit has been lowered because linear allocation was disabled
            // or because incremental marking wants to get a chance to do a step,
            // or because idle scavenge job wants to get a chance to post a task.
            // Set the new limit accordingly.
            Address new_top = old_top + aligned_size_in_bytes;
            Address soon_object = old_top + filler_size;
            InlineAllocationStep(new_top, new_top, soon_object, size_in_bytes);
            UpdateInlineAllocationLimit(aligned_size_in_bytes);
        }
        return true;
    }

    size_t LargeObjectSpace::Available()
    {
        // We return zero here since we cannot take advantage of already allocated
        // large object memory.
        return 0;
    }

    void SpaceWithLinearArea::StartNextInlineAllocationStep()
    {
        if (heap()->allocation_step_in_progress()) {
            // If we are mid-way through an existing step, don't start a new one.
            return;
        }

        if (AllocationObserversActive()) {
            top_on_previous_step_ = top();
            UpdateInlineAllocationLimit(0);
        } else {
            DCHECK_EQ(kNullAddress, top_on_previous_step_);
        }
    }

    void SpaceWithLinearArea::AddAllocationObserver(AllocationObserver* observer)
    {
        InlineAllocationStep(top(), top(), kNullAddress, 0);
        Space::AddAllocationObserver(observer);
        DCHECK_IMPLIES(top_on_previous_step_, AllocationObserversActive());
    }

    void SpaceWithLinearArea::RemoveAllocationObserver(
        AllocationObserver* observer)
    {
        Address top_for_next_step = allocation_observers_->size() == 1 ? kNullAddress : top();
        InlineAllocationStep(top(), top_for_next_step, kNullAddress, 0);
        Space::RemoveAllocationObserver(observer);
        DCHECK_IMPLIES(top_on_previous_step_, AllocationObserversActive());
    }

    void SpaceWithLinearArea::PauseAllocationObservers()
    {
        // Do a step to account for memory allocated so far.
        InlineAllocationStep(top(), kNullAddress, kNullAddress, 0);
        Space::PauseAllocationObservers();
        DCHECK_EQ(kNullAddress, top_on_previous_step_);
        UpdateInlineAllocationLimit(0);
    }

    void SpaceWithLinearArea::ResumeAllocationObservers()
    {
        DCHECK_EQ(kNullAddress, top_on_previous_step_);
        Space::ResumeAllocationObservers();
        StartNextInlineAllocationStep();
    }

    void SpaceWithLinearArea::InlineAllocationStep(Address top,
        Address top_for_next_step,
        Address soon_object,
        size_t size)
    {
        if (heap()->allocation_step_in_progress()) {
            // Avoid starting a new step if we are mid-way through an existing one.
            return;
        }

        if (top_on_previous_step_) {
            if (top < top_on_previous_step_) {
                // Generated code decreased the top pointer to do folded allocations.
                DCHECK_NE(top, kNullAddress);
                DCHECK_EQ(Page::FromAllocationAreaAddress(top),
                    Page::FromAllocationAreaAddress(top_on_previous_step_));
                top_on_previous_step_ = top;
            }
            int bytes_allocated = static_cast<int>(top - top_on_previous_step_);
            AllocationStep(bytes_allocated, soon_object, static_cast<int>(size));
            top_on_previous_step_ = top_for_next_step;
        }
    }

    std::unique_ptr<ObjectIterator> NewSpace::GetObjectIterator()
    {
        return std::unique_ptr<ObjectIterator>(new SemiSpaceIterator(this));
    }

#ifdef VERIFY_HEAP
    // We do not use the SemiSpaceIterator because verification doesn't assume
    // that it works (it depends on the invariants we are checking).
    void NewSpace::Verify(Isolate* isolate)
    {
        // The allocation pointer should be in the space or at the very end.
        DCHECK_SEMISPACE_ALLOCATION_INFO(allocation_info_, to_space_);

        // There should be objects packed in from the low address up to the
        // allocation pointer.
        Address current = to_space_.first_page()->area_start();
        CHECK_EQ(current, to_space_.space_start());

        size_t external_space_bytes[kNumTypes];
        for (int i = 0; i < kNumTypes; i++) {
            external_space_bytes[static_cast<ExternalBackingStoreType>(i)] = 0;
        }

        while (current != top()) {
            if (!Page::IsAlignedToPageSize(current)) {
                // The allocation pointer should not be in the middle of an object.
                CHECK(!Page::FromAllocationAreaAddress(current)->ContainsLimit(top()) || current < top());

                HeapObject object = HeapObject::FromAddress(current);

                // The first word should be a map, and we expect all map pointers to
                // be in map space or read-only space.
                Map map = object->map();
                CHECK(map->IsMap());
                CHECK(heap()->map_space()->Contains(map) || heap()->read_only_space()->Contains(map));

                // The object should not be code or a map.
                CHECK(!object->IsMap());
                CHECK(!object->IsAbstractCode());

                // The object itself should look OK.
                object->ObjectVerify(isolate);

                // All the interior pointers should be contained in the heap.
                VerifyPointersVisitor visitor(heap());
                int size = object->Size();
                object->IterateBody(map, size, &visitor);

                if (object->IsExternalString()) {
                    ExternalString external_string = ExternalString::cast(object);
                    size_t size = external_string->ExternalPayloadSize();
                    external_space_bytes[ExternalBackingStoreType::kExternalString] += size;
                } else if (object->IsJSArrayBuffer()) {
                    JSArrayBuffer array_buffer = JSArrayBuffer::cast(object);
                    if (ArrayBufferTracker::IsTracked(array_buffer)) {
                        size_t size = array_buffer->byte_length();
                        external_space_bytes[ExternalBackingStoreType::kArrayBuffer] += size;
                    }
                }

                current += size;
            } else {
                // At end of page, switch to next page.
                Page* page = Page::FromAllocationAreaAddress(current)->next_page();
                current = page->area_start();
            }
        }

        for (int i = 0; i < kNumTypes; i++) {
            ExternalBackingStoreType t = static_cast<ExternalBackingStoreType>(i);
            CHECK_EQ(external_space_bytes[t], ExternalBackingStoreBytes(t));
        }

        // Check semi-spaces.
        CHECK_EQ(from_space_.id(), kFromSpace);
        CHECK_EQ(to_space_.id(), kToSpace);
        from_space_.Verify();
        to_space_.Verify();
    }
#endif

    // -----------------------------------------------------------------------------
    // SemiSpace implementation

    void SemiSpace::SetUp(size_t initial_capacity, size_t maximum_capacity)
    {
        DCHECK_GE(maximum_capacity, static_cast<size_t>(Page::kPageSize));
        minimum_capacity_ = RoundDown(initial_capacity, Page::kPageSize);
        current_capacity_ = minimum_capacity_;
        maximum_capacity_ = RoundDown(maximum_capacity, Page::kPageSize);
        committed_ = false;
    }

    void SemiSpace::TearDown()
    {
        // Properly uncommit memory to keep the allocator counters in sync.
        if (is_committed()) {
            Uncommit();
        }
        current_capacity_ = maximum_capacity_ = 0;
    }

    bool SemiSpace::Commit()
    {
        DCHECK(!is_committed());
        const int num_pages = static_cast<int>(current_capacity_ / Page::kPageSize);
        for (int pages_added = 0; pages_added < num_pages; pages_added++) {
            Page* new_page = heap()->memory_allocator()->AllocatePage<MemoryAllocator::kPooled>(
                MemoryChunkLayout::AllocatableMemoryInDataPage(), this,
                NOT_EXECUTABLE);
            if (new_page == nullptr) {
                if (pages_added)
                    RewindPages(pages_added);
                return false;
            }
            memory_chunk_list_.PushBack(new_page);
        }
        Reset();
        AccountCommitted(current_capacity_);
        if (age_mark_ == kNullAddress) {
            age_mark_ = first_page()->area_start();
        }
        committed_ = true;
        return true;
    }

    bool SemiSpace::Uncommit()
    {
        DCHECK(is_committed());
        while (!memory_chunk_list_.Empty()) {
            MemoryChunk* chunk = memory_chunk_list_.front();
            memory_chunk_list_.Remove(chunk);
            heap()->memory_allocator()->Free<MemoryAllocator::kPooledAndQueue>(chunk);
        }
        current_page_ = nullptr;
        AccountUncommitted(current_capacity_);
        committed_ = false;
        heap()->memory_allocator()->unmapper()->FreeQueuedChunks();
        return true;
    }

    size_t SemiSpace::CommittedPhysicalMemory()
    {
        if (!is_committed())
            return 0;
        size_t size = 0;
        for (Page* p : *this) {
            size += p->CommittedPhysicalMemory();
        }
        return size;
    }

    bool SemiSpace::GrowTo(size_t new_capacity)
    {
        if (!is_committed()) {
            if (!Commit())
                return false;
        }
        DCHECK_EQ(new_capacity & kPageAlignmentMask, 0u);
        DCHECK_LE(new_capacity, maximum_capacity_);
        DCHECK_GT(new_capacity, current_capacity_);
        const size_t delta = new_capacity - current_capacity_;
        DCHECK(IsAligned(delta, AllocatePageSize()));
        const int delta_pages = static_cast<int>(delta / Page::kPageSize);
        DCHECK(last_page());
        IncrementalMarking::NonAtomicMarkingState* marking_state = heap()->incremental_marking()->non_atomic_marking_state();
        for (int pages_added = 0; pages_added < delta_pages; pages_added++) {
            Page* new_page = heap()->memory_allocator()->AllocatePage<MemoryAllocator::kPooled>(
                MemoryChunkLayout::AllocatableMemoryInDataPage(), this,
                NOT_EXECUTABLE);
            if (new_page == nullptr) {
                if (pages_added)
                    RewindPages(pages_added);
                return false;
            }
            memory_chunk_list_.PushBack(new_page);
            marking_state->ClearLiveness(new_page);
            // Duplicate the flags that was set on the old page.
            new_page->SetFlags(last_page()->GetFlags(), Page::kCopyOnFlipFlagsMask);
        }
        AccountCommitted(delta);
        current_capacity_ = new_capacity;
        return true;
    }

    void SemiSpace::RewindPages(int num_pages)
    {
        DCHECK_GT(num_pages, 0);
        DCHECK(last_page());
        while (num_pages > 0) {
            MemoryChunk* last = last_page();
            memory_chunk_list_.Remove(last);
            heap()->memory_allocator()->Free<MemoryAllocator::kPooledAndQueue>(last);
            num_pages--;
        }
    }

    bool SemiSpace::ShrinkTo(size_t new_capacity)
    {
        DCHECK_EQ(new_capacity & kPageAlignmentMask, 0u);
        DCHECK_GE(new_capacity, minimum_capacity_);
        DCHECK_LT(new_capacity, current_capacity_);
        if (is_committed()) {
            const size_t delta = current_capacity_ - new_capacity;
            DCHECK(IsAligned(delta, Page::kPageSize));
            int delta_pages = static_cast<int>(delta / Page::kPageSize);
            RewindPages(delta_pages);
            AccountUncommitted(delta);
            heap()->memory_allocator()->unmapper()->FreeQueuedChunks();
        }
        current_capacity_ = new_capacity;
        return true;
    }

    void SemiSpace::FixPagesFlags(intptr_t flags, intptr_t mask)
    {
        for (Page* page : *this) {
            page->set_owner(this);
            page->SetFlags(flags, mask);
            if (id_ == kToSpace) {
                page->ClearFlag(MemoryChunk::FROM_PAGE);
                page->SetFlag(MemoryChunk::TO_PAGE);
                page->ClearFlag(MemoryChunk::NEW_SPACE_BELOW_AGE_MARK);
                heap()->incremental_marking()->non_atomic_marking_state()->SetLiveBytes(
                    page, 0);
            } else {
                page->SetFlag(MemoryChunk::FROM_PAGE);
                page->ClearFlag(MemoryChunk::TO_PAGE);
            }
            DCHECK(page->InYoungGeneration());
        }
    }

    void SemiSpace::Reset()
    {
        DCHECK(first_page());
        DCHECK(last_page());
        current_page_ = first_page();
        pages_used_ = 0;
    }

    void SemiSpace::RemovePage(Page* page)
    {
        if (current_page_ == page) {
            if (page->prev_page()) {
                current_page_ = page->prev_page();
            }
        }
        memory_chunk_list_.Remove(page);
        for (size_t i = 0; i < ExternalBackingStoreType::kNumTypes; i++) {
            ExternalBackingStoreType t = static_cast<ExternalBackingStoreType>(i);
            DecrementExternalBackingStoreBytes(t, page->ExternalBackingStoreBytes(t));
        }
    }

    void SemiSpace::PrependPage(Page* page)
    {
        page->SetFlags(current_page()->GetFlags(),
            static_cast<uintptr_t>(Page::kCopyAllFlags));
        page->set_owner(this);
        memory_chunk_list_.PushFront(page);
        pages_used_++;
        for (size_t i = 0; i < ExternalBackingStoreType::kNumTypes; i++) {
            ExternalBackingStoreType t = static_cast<ExternalBackingStoreType>(i);
            IncrementExternalBackingStoreBytes(t, page->ExternalBackingStoreBytes(t));
        }
    }

    void SemiSpace::Swap(SemiSpace* from, SemiSpace* to)
    {
        // We won't be swapping semispaces without data in them.
        DCHECK(from->first_page());
        DCHECK(to->first_page());

        intptr_t saved_to_space_flags = to->current_page()->GetFlags();

        // We swap all properties but id_.
        std::swap(from->current_capacity_, to->current_capacity_);
        std::swap(from->maximum_capacity_, to->maximum_capacity_);
        std::swap(from->minimum_capacity_, to->minimum_capacity_);
        std::swap(from->age_mark_, to->age_mark_);
        std::swap(from->committed_, to->committed_);
        std::swap(from->memory_chunk_list_, to->memory_chunk_list_);
        std::swap(from->current_page_, to->current_page_);
        std::swap(from->external_backing_store_bytes_,
            to->external_backing_store_bytes_);

        to->FixPagesFlags(saved_to_space_flags, Page::kCopyOnFlipFlagsMask);
        from->FixPagesFlags(0, 0);
    }

    void SemiSpace::set_age_mark(Address mark)
    {
        DCHECK_EQ(Page::FromAllocationAreaAddress(mark)->owner(), this);
        age_mark_ = mark;
        // Mark all pages up to the one containing mark.
        for (Page* p : PageRange(space_start(), mark)) {
            p->SetFlag(MemoryChunk::NEW_SPACE_BELOW_AGE_MARK);
        }
    }

    std::unique_ptr<ObjectIterator> SemiSpace::GetObjectIterator()
    {
        // Use the NewSpace::NewObjectIterator to iterate the ToSpace.
        UNREACHABLE();
    }

#ifdef DEBUG
    void SemiSpace::Print()
    {
    }
#endif

#ifdef VERIFY_HEAP
    void SemiSpace::Verify()
    {
        bool is_from_space = (id_ == kFromSpace);
        size_t external_backing_store_bytes[kNumTypes];

        for (int i = 0; i < kNumTypes; i++) {
            external_backing_store_bytes[static_cast<ExternalBackingStoreType>(i)] = 0;
        }

        for (Page* page : *this) {
            CHECK_EQ(page->owner(), this);
            CHECK(page->InNewSpace());
            CHECK(page->IsFlagSet(is_from_space ? MemoryChunk::FROM_PAGE
                                                : MemoryChunk::TO_PAGE));
            CHECK(!page->IsFlagSet(is_from_space ? MemoryChunk::TO_PAGE
                                                 : MemoryChunk::FROM_PAGE));
            CHECK(page->IsFlagSet(MemoryChunk::POINTERS_TO_HERE_ARE_INTERESTING));
            if (!is_from_space) {
                // The pointers-from-here-are-interesting flag isn't updated dynamically
                // on from-space pages, so it might be out of sync with the marking state.
                if (page->heap()->incremental_marking()->IsMarking()) {
                    CHECK(page->IsFlagSet(MemoryChunk::POINTERS_FROM_HERE_ARE_INTERESTING));
                } else {
                    CHECK(
                        !page->IsFlagSet(MemoryChunk::POINTERS_FROM_HERE_ARE_INTERESTING));
                }
            }
            for (int i = 0; i < kNumTypes; i++) {
                ExternalBackingStoreType t = static_cast<ExternalBackingStoreType>(i);
                external_backing_store_bytes[t] += page->ExternalBackingStoreBytes(t);
            }

            CHECK_IMPLIES(page->list_node().prev(),
                page->list_node().prev()->list_node().next() == page);
        }
        for (int i = 0; i < kNumTypes; i++) {
            ExternalBackingStoreType t = static_cast<ExternalBackingStoreType>(i);
            CHECK_EQ(external_backing_store_bytes[t], ExternalBackingStoreBytes(t));
        }
    }
#endif

#ifdef DEBUG
    void SemiSpace::AssertValidRange(Address start, Address end)
    {
        // Addresses belong to same semi-space
        Page* page = Page::FromAllocationAreaAddress(start);
        Page* end_page = Page::FromAllocationAreaAddress(end);
        SemiSpace* space = reinterpret_cast<SemiSpace*>(page->owner());
        DCHECK_EQ(space, end_page->owner());
        // Start address is before end address, either on same page,
        // or end address is on a later page in the linked list of
        // semi-space pages.
        if (page == end_page) {
            DCHECK_LE(start, end);
        } else {
            while (page != end_page) {
                page = page->next_page();
            }
            DCHECK(page);
        }
    }
#endif

    // -----------------------------------------------------------------------------
    // SemiSpaceIterator implementation.

    SemiSpaceIterator::SemiSpaceIterator(NewSpace* space)
    {
        Initialize(space->first_allocatable_address(), space->top());
    }

    void SemiSpaceIterator::Initialize(Address start, Address end)
    {
        SemiSpace::AssertValidRange(start, end);
        current_ = start;
        limit_ = end;
    }

    size_t NewSpace::CommittedPhysicalMemory()
    {
        if (!base::OS::HasLazyCommits())
            return CommittedMemory();
        MemoryChunk::UpdateHighWaterMark(allocation_info_.top());
        size_t size = to_space_.CommittedPhysicalMemory();
        if (from_space_.is_committed()) {
            size += from_space_.CommittedPhysicalMemory();
        }
        return size;
    }

    // -----------------------------------------------------------------------------
    // Free lists for old object spaces implementation

    void FreeListCategory::Reset()
    {
        set_top(FreeSpace());
        set_prev(nullptr);
        set_next(nullptr);
        available_ = 0;
    }

    FreeSpace FreeListCategory::PickNodeFromList(size_t minimum_size,
        size_t* node_size)
    {
        DCHECK(page()->CanAllocate());
        FreeSpace node = top();
        if (node.is_null() || static_cast<size_t>(node->Size()) < minimum_size) {
            *node_size = 0;
            return FreeSpace();
        }
        set_top(node->next());
        *node_size = node->Size();
        available_ -= *node_size;
        return node;
    }

    FreeSpace FreeListCategory::SearchForNodeInList(size_t minimum_size,
        size_t* node_size)
    {
        DCHECK(page()->CanAllocate());
        FreeSpace prev_non_evac_node;
        for (FreeSpace cur_node = top(); !cur_node.is_null();
             cur_node = cur_node->next()) {
            size_t size = cur_node->size();
            if (size >= minimum_size) {
                DCHECK_GE(available_, size);
                available_ -= size;
                if (cur_node == top()) {
                    set_top(cur_node->next());
                }
                if (!prev_non_evac_node.is_null()) {
                    MemoryChunk* chunk = MemoryChunk::FromHeapObject(prev_non_evac_node);
                    if (chunk->owner()->identity() == CODE_SPACE) {
                        chunk->heap()->UnprotectAndRegisterMemoryChunk(chunk);
                    }
                    prev_non_evac_node->set_next(cur_node->next());
                }
                *node_size = size;
                return cur_node;
            }

            prev_non_evac_node = cur_node;
        }
        return FreeSpace();
    }

    void FreeListCategory::Free(Address start, size_t size_in_bytes,
        FreeMode mode)
    {
        FreeSpace free_space = FreeSpace::cast(HeapObject::FromAddress(start));
        free_space->set_next(top());
        set_top(free_space);
        available_ += size_in_bytes;
        if ((mode == kLinkCategory) && (prev() == nullptr) && (next() == nullptr)) {
            owner()->AddCategory(this);
        }
    }

    void FreeListCategory::RepairFreeList(Heap* heap)
    {
        FreeSpace n = top();
        while (!n.is_null()) {
            MapWordSlot map_location = n.map_slot();
            // We can't use .is_null() here because *map_location returns an
            // Object (for which "is null" is not defined, as it would be
            // indistinguishable from "is Smi(0)"). Only HeapObject has "is_null()".
            if (map_location.contains_value(kNullAddress)) {
                map_location.store(ReadOnlyRoots(heap).free_space_map());
            } else {
                DCHECK(map_location.contains_value(
                    ReadOnlyRoots(heap).free_space_map().ptr()));
            }
            n = n->next();
        }
    }

    void FreeListCategory::Relink()
    {
        DCHECK(!is_linked());
        owner()->AddCategory(this);
    }

    FreeList::FreeList()
        : wasted_bytes_(0)
    {
        for (int i = kFirstCategory; i < kNumberOfCategories; i++) {
            categories_[i] = nullptr;
        }
        Reset();
    }

    void FreeList::Reset()
    {
        ForAllFreeListCategories(
            [](FreeListCategory* category) { category->Reset(); });
        for (int i = kFirstCategory; i < kNumberOfCategories; i++) {
            categories_[i] = nullptr;
        }
        ResetStats();
    }

    size_t FreeList::Free(Address start, size_t size_in_bytes, FreeMode mode)
    {
        Page* page = Page::FromAddress(start);
        page->DecreaseAllocatedBytes(size_in_bytes);

        // Blocks have to be a minimum size to hold free list items.
        if (size_in_bytes < kMinBlockSize) {
            page->add_wasted_memory(size_in_bytes);
            wasted_bytes_ += size_in_bytes;
            return size_in_bytes;
        }

        // Insert other blocks at the head of a free list of the appropriate
        // magnitude.
        FreeListCategoryType type = SelectFreeListCategoryType(size_in_bytes);
        page->free_list_category(type)->Free(start, size_in_bytes, mode);
        DCHECK_EQ(page->AvailableInFreeList(),
            page->AvailableInFreeListFromAllocatedBytes());
        return 0;
    }

    FreeSpace FreeList::FindNodeIn(FreeListCategoryType type, size_t minimum_size,
        size_t* node_size)
    {
        FreeListCategoryIterator it(this, type);
        FreeSpace node;
        while (it.HasNext()) {
            FreeListCategory* current = it.Next();
            node = current->PickNodeFromList(minimum_size, node_size);
            if (!node.is_null()) {
                DCHECK(IsVeryLong() || Available() == SumFreeLists());
                return node;
            }
            RemoveCategory(current);
        }
        return node;
    }

    FreeSpace FreeList::TryFindNodeIn(FreeListCategoryType type,
        size_t minimum_size, size_t* node_size)
    {
        if (categories_[type] == nullptr)
            return FreeSpace();
        FreeSpace node = categories_[type]->PickNodeFromList(minimum_size, node_size);
        if (!node.is_null()) {
            DCHECK(IsVeryLong() || Available() == SumFreeLists());
        }
        return node;
    }

    FreeSpace FreeList::SearchForNodeInList(FreeListCategoryType type,
        size_t* node_size,
        size_t minimum_size)
    {
        FreeListCategoryIterator it(this, type);
        FreeSpace node;
        while (it.HasNext()) {
            FreeListCategory* current = it.Next();
            node = current->SearchForNodeInList(minimum_size, node_size);
            if (!node.is_null()) {
                DCHECK(IsVeryLong() || Available() == SumFreeLists());
                return node;
            }
            if (current->is_empty()) {
                RemoveCategory(current);
            }
        }
        return node;
    }

    FreeSpace FreeList::Allocate(size_t size_in_bytes, size_t* node_size)
    {
        DCHECK_GE(kMaxBlockSize, size_in_bytes);
        FreeSpace node;
        // First try the allocation fast path: try to allocate the minimum element
        // size of a free list category. This operation is constant time.
        FreeListCategoryType type = SelectFastAllocationFreeListCategoryType(size_in_bytes);
        for (int i = type; i < kHuge && node.is_null(); i++) {
            node = FindNodeIn(static_cast<FreeListCategoryType>(i), size_in_bytes,
                node_size);
        }

        if (node.is_null()) {
            // Next search the huge list for free list nodes. This takes linear time in
            // the number of huge elements.
            node = SearchForNodeInList(kHuge, node_size, size_in_bytes);
        }

        if (node.is_null() && type != kHuge) {
            // We didn't find anything in the huge list. Now search the best fitting
            // free list for a node that has at least the requested size.
            type = SelectFreeListCategoryType(size_in_bytes);
            node = TryFindNodeIn(type, size_in_bytes, node_size);
        }

        if (!node.is_null()) {
            Page::FromHeapObject(node)->IncreaseAllocatedBytes(*node_size);
        }

        DCHECK(IsVeryLong() || Available() == SumFreeLists());
        return node;
    }

    size_t FreeList::EvictFreeListItems(Page* page)
    {
        size_t sum = 0;
        page->ForAllFreeListCategories([this, &sum](FreeListCategory* category) {
            DCHECK_EQ(this, category->owner());
            sum += category->available();
            RemoveCategory(category);
            category->Reset();
        });
        return sum;
    }

    bool FreeList::ContainsPageFreeListItems(Page* page)
    {
        bool contained = false;
        page->ForAllFreeListCategories(
            [this, &contained](FreeListCategory* category) {
                if (category->owner() == this && category->is_linked()) {
                    contained = true;
                }
            });
        return contained;
    }

    void FreeList::RepairLists(Heap* heap)
    {
        ForAllFreeListCategories(
            [heap](FreeListCategory* category) { category->RepairFreeList(heap); });
    }

    bool FreeList::AddCategory(FreeListCategory* category)
    {
        FreeListCategoryType type = category->type_;
        DCHECK_LT(type, kNumberOfCategories);
        FreeListCategory* top = categories_[type];

        if (category->is_empty())
            return false;
        if (top == category)
            return false;

        // Common double-linked list insertion.
        if (top != nullptr) {
            top->set_prev(category);
        }
        category->set_next(top);
        categories_[type] = category;
        return true;
    }

    void FreeList::RemoveCategory(FreeListCategory* category)
    {
        FreeListCategoryType type = category->type_;
        DCHECK_LT(type, kNumberOfCategories);
        FreeListCategory* top = categories_[type];

        // Common double-linked list removal.
        if (top == category) {
            categories_[type] = category->next();
        }
        if (category->prev() != nullptr) {
            category->prev()->set_next(category->next());
        }
        if (category->next() != nullptr) {
            category->next()->set_prev(category->prev());
        }
        category->set_next(nullptr);
        category->set_prev(nullptr);
    }

    void FreeList::PrintCategories(FreeListCategoryType type)
    {
        FreeListCategoryIterator it(this, type);
        PrintF("FreeList[%p, top=%p, %d] ", static_cast<void*>(this),
            static_cast<void*>(categories_[type]), type);
        while (it.HasNext()) {
            FreeListCategory* current = it.Next();
            PrintF("%p -> ", static_cast<void*>(current));
        }
        PrintF("null\n");
    }

#ifdef DEBUG
    size_t FreeListCategory::SumFreeList()
    {
        size_t sum = 0;
        FreeSpace cur = top();
        while (!cur.is_null()) {
            // We can't use "cur->map()" here because both cur's map and the
            // root can be null during bootstrapping.
            DCHECK(cur->map_slot().contains_value(
                page()->heap()->isolate()->root(RootIndex::kFreeSpaceMap).ptr()));
            sum += cur->relaxed_read_size();
            cur = cur->next();
        }
        return sum;
    }

    int FreeListCategory::FreeListLength()
    {
        int length = 0;
        FreeSpace cur = top();
        while (!cur.is_null()) {
            length++;
            cur = cur->next();
            if (length == kVeryLongFreeList)
                return length;
        }
        return length;
    }

    bool FreeList::IsVeryLong()
    {
        int len = 0;
        for (int i = kFirstCategory; i < kNumberOfCategories; i++) {
            FreeListCategoryIterator it(this, static_cast<FreeListCategoryType>(i));
            while (it.HasNext()) {
                len += it.Next()->FreeListLength();
                if (len >= FreeListCategory::kVeryLongFreeList)
                    return true;
            }
        }
        return false;
    }

    // This can take a very long time because it is linear in the number of entries
    // on the free list, so it should not be called if FreeListLength returns
    // kVeryLongFreeList.
    size_t FreeList::SumFreeLists()
    {
        size_t sum = 0;
        ForAllFreeListCategories(
            [&sum](FreeListCategory* category) { sum += category->SumFreeList(); });
        return sum;
    }
#endif

    // -----------------------------------------------------------------------------
    // OldSpace implementation

    void PagedSpace::PrepareForMarkCompact()
    {
        // We don't have a linear allocation area while sweeping.  It will be restored
        // on the first allocation after the sweep.
        FreeLinearAllocationArea();

        // Clear the free list before a full GC---it will be rebuilt afterward.
        free_list_.Reset();
    }

    size_t PagedSpace::SizeOfObjects()
    {
        CHECK_GE(limit(), top());
        DCHECK_GE(Size(), static_cast<size_t>(limit() - top()));
        return Size() - (limit() - top());
    }

    bool PagedSpace::SweepAndRetryAllocation(int size_in_bytes)
    {
        MarkCompactCollector* collector = heap()->mark_compact_collector();
        if (collector->sweeping_in_progress()) {
            // Wait for the sweeper threads here and complete the sweeping phase.
            collector->EnsureSweepingCompleted();

            // After waiting for the sweeper threads, there may be new free-list
            // entries.
            return RefillLinearAllocationAreaFromFreeList(size_in_bytes);
        }
        return false;
    }

    bool CompactionSpace::SweepAndRetryAllocation(int size_in_bytes)
    {
        MarkCompactCollector* collector = heap()->mark_compact_collector();
        if (FLAG_concurrent_sweeping && collector->sweeping_in_progress()) {
            collector->sweeper()->ParallelSweepSpace(identity(), 0);
            RefillFreeList();
            return RefillLinearAllocationAreaFromFreeList(size_in_bytes);
        }
        return false;
    }

    bool PagedSpace::SlowRefillLinearAllocationArea(int size_in_bytes)
    {
        VMState<GC> state(heap()->isolate());
        RuntimeCallTimerScope runtime_timer(
            heap()->isolate(), RuntimeCallCounterId::kGC_Custom_SlowAllocateRaw);
        return RawSlowRefillLinearAllocationArea(size_in_bytes);
    }

    bool CompactionSpace::SlowRefillLinearAllocationArea(int size_in_bytes)
    {
        return RawSlowRefillLinearAllocationArea(size_in_bytes);
    }

    bool PagedSpace::RawSlowRefillLinearAllocationArea(int size_in_bytes)
    {
        // Allocation in this space has failed.
        DCHECK_GE(size_in_bytes, 0);
        const int kMaxPagesToSweep = 1;

        if (RefillLinearAllocationAreaFromFreeList(size_in_bytes))
            return true;

        MarkCompactCollector* collector = heap()->mark_compact_collector();
        // Sweeping is still in progress.
        if (collector->sweeping_in_progress()) {
            if (FLAG_concurrent_sweeping && !is_local() && !collector->sweeper()->AreSweeperTasksRunning()) {
                collector->EnsureSweepingCompleted();
            }

            // First try to refill the free-list, concurrent sweeper threads
            // may have freed some objects in the meantime.
            RefillFreeList();

            // Retry the free list allocation.
            if (RefillLinearAllocationAreaFromFreeList(
                    static_cast<size_t>(size_in_bytes)))
                return true;

            // If sweeping is still in progress try to sweep pages.
            int max_freed = collector->sweeper()->ParallelSweepSpace(
                identity(), size_in_bytes, kMaxPagesToSweep);
            RefillFreeList();
            if (max_freed >= size_in_bytes) {
                if (RefillLinearAllocationAreaFromFreeList(
                        static_cast<size_t>(size_in_bytes)))
                    return true;
            }
        }

        if (is_local()) {
            // The main thread may have acquired all swept pages. Try to steal from
            // it. This can only happen during young generation evacuation.
            PagedSpace* main_space = heap()->paged_space(identity());
            Page* page = main_space->RemovePageSafe(size_in_bytes);
            if (page != nullptr) {
                AddPage(page);
                if (RefillLinearAllocationAreaFromFreeList(
                        static_cast<size_t>(size_in_bytes)))
                    return true;
            }
        }

        if (heap()->ShouldExpandOldGenerationOnSlowAllocation() && Expand()) {
            DCHECK((CountTotalPages() > 1) || (static_cast<size_t>(size_in_bytes) <= free_list_.Available()));
            return RefillLinearAllocationAreaFromFreeList(
                static_cast<size_t>(size_in_bytes));
        }

        // If sweeper threads are active, wait for them at that point and steal
        // elements form their free-lists. Allocation may still fail their which
        // would indicate that there is not enough memory for the given allocation.
        return SweepAndRetryAllocation(size_in_bytes);
    }

    // -----------------------------------------------------------------------------
    // MapSpace implementation

#ifdef VERIFY_HEAP
    void MapSpace::VerifyObject(HeapObject object)
    {
        CHECK(object->IsMap());
    }
#endif

    ReadOnlySpace::ReadOnlySpace(Heap* heap)
        : PagedSpace(heap, RO_SPACE, NOT_EXECUTABLE)
        , is_string_padding_cleared_(heap->isolate()->initialized_from_snapshot())
    {
    }

    void ReadOnlyPage::MakeHeaderRelocatable()
    {
        if (mutex_ != nullptr) {
            // TODO(v8:7464): heap_ and owner_ need to be cleared as well.
            delete mutex_;
            mutex_ = nullptr;
            local_tracker_ = nullptr;
            reservation_.Reset();
        }
    }

    void ReadOnlySpace::Forget()
    {
        for (Page* p : *this) {
            heap()->memory_allocator()->PreFreeMemory(p);
        }
    }

    void ReadOnlySpace::SetPermissionsForPages(PageAllocator::Permission access)
    {
        MemoryAllocator* memory_allocator = heap()->memory_allocator();
        for (Page* p : *this) {
            ReadOnlyPage* page = static_cast<ReadOnlyPage*>(p);
            if (access == PageAllocator::kRead) {
                page->MakeHeaderRelocatable();
            }

            // Read only pages don't have valid reservation object so we get proper
            // page allocator manually.
            v8::PageAllocator* page_allocator = memory_allocator->page_allocator(page->executable());
            CHECK(
                SetPermissions(page_allocator, page->address(), page->size(), access));
        }
    }

    // After we have booted, we have created a map which represents free space
    // on the heap.  If there was already a free list then the elements on it
    // were created with the wrong FreeSpaceMap (normally nullptr), so we need to
    // fix them.
    void ReadOnlySpace::RepairFreeListsAfterDeserialization()
    {
        free_list_.RepairLists(heap());
        // Each page may have a small free space that is not tracked by a free list.
        // Those free spaces still contain null as their map pointer.
        // Overwrite them with new fillers.
        for (Page* page : *this) {
            int size = static_cast<int>(page->wasted_memory());
            if (size == 0) {
                // If there is no wasted memory then all free space is in the free list.
                continue;
            }
            Address start = page->HighWaterMark();
            Address end = page->area_end();
            if (start < end - size) {
                // A region at the high watermark is already in free list.
                HeapObject filler = HeapObject::FromAddress(start);
                CHECK(filler->IsFiller());
                start += filler->Size();
            }
            CHECK_EQ(size, static_cast<int>(end - start));
            heap()->CreateFillerObjectAt(start, size, ClearRecordedSlots::kNo);
        }
    }

    void ReadOnlySpace::ClearStringPaddingIfNeeded()
    {
        if (is_string_padding_cleared_)
            return;

        WritableScope writable_scope(this);
        for (Page* page : *this) {
            HeapObjectIterator iterator(page);
            for (HeapObject o = iterator.Next(); !o.is_null(); o = iterator.Next()) {
                if (o->IsSeqOneByteString()) {
                    SeqOneByteString::cast(o)->clear_padding();
                } else if (o->IsSeqTwoByteString()) {
                    SeqTwoByteString::cast(o)->clear_padding();
                }
            }
        }
        is_string_padding_cleared_ = true;
    }

    void ReadOnlySpace::MarkAsReadOnly()
    {
        DCHECK(!is_marked_read_only_);
        FreeLinearAllocationArea();
        is_marked_read_only_ = true;
        SetPermissionsForPages(PageAllocator::kRead);
    }

    void ReadOnlySpace::MarkAsReadWrite()
    {
        DCHECK(is_marked_read_only_);
        SetPermissionsForPages(PageAllocator::kReadWrite);
        is_marked_read_only_ = false;
    }

    Address LargePage::GetAddressToShrink(Address object_address,
        size_t object_size)
    {
        if (executable() == EXECUTABLE) {
            return 0;
        }
        size_t used_size = ::RoundUp((object_address - address()) + object_size,
            MemoryAllocator::GetCommitPageSize());
        if (used_size < CommittedPhysicalMemory()) {
            return address() + used_size;
        }
        return 0;
    }

    void LargePage::ClearOutOfLiveRangeSlots(Address free_start)
    {
        RememberedSet<OLD_TO_NEW>::RemoveRange(this, free_start, area_end(),
            SlotSet::FREE_EMPTY_BUCKETS);
        RememberedSet<OLD_TO_OLD>::RemoveRange(this, free_start, area_end(),
            SlotSet::FREE_EMPTY_BUCKETS);
        RememberedSet<OLD_TO_NEW>::RemoveRangeTyped(this, free_start, area_end());
        RememberedSet<OLD_TO_OLD>::RemoveRangeTyped(this, free_start, area_end());
    }

    // -----------------------------------------------------------------------------
    // LargeObjectIterator

    LargeObjectIterator::LargeObjectIterator(LargeObjectSpace* space)
    {
        current_ = space->first_page();
    }

    HeapObject LargeObjectIterator::Next()
    {
        if (current_ == nullptr)
            return HeapObject();

        HeapObject object = current_->GetObject();
        current_ = current_->next_page();
        return object;
    }

    // -----------------------------------------------------------------------------
    // LargeObjectSpace

    LargeObjectSpace::LargeObjectSpace(Heap* heap)
        : LargeObjectSpace(heap, LO_SPACE)
    {
    }

    LargeObjectSpace::LargeObjectSpace(Heap* heap, AllocationSpace id)
        : Space(heap, id)
        , size_(0)
        , page_count_(0)
        , objects_size_(0)
    {
    }

    void LargeObjectSpace::TearDown()
    {
        while (!memory_chunk_list_.Empty()) {
            LargePage* page = first_page();
            LOG(heap()->isolate(),
                DeleteEvent("LargeObjectChunk",
                    reinterpret_cast<void*>(page->address())));
            memory_chunk_list_.Remove(page);
            heap()->memory_allocator()->Free<MemoryAllocator::kFull>(page);
        }
    }

    AllocationResult LargeObjectSpace::AllocateRaw(int object_size)
    {
        return AllocateRaw(object_size, NOT_EXECUTABLE);
    }

    AllocationResult LargeObjectSpace::AllocateRaw(int object_size,
        Executability executable)
    {
        // Check if we want to force a GC before growing the old space further.
        // If so, fail the allocation.
        if (!heap()->CanExpandOldGeneration(object_size) || !heap()->ShouldExpandOldGenerationOnSlowAllocation()) {
            return AllocationResult::Retry(identity());
        }

        LargePage* page = AllocateLargePage(object_size, executable);
        if (page == nullptr)
            return AllocationResult::Retry(identity());
        page->SetOldGenerationPageFlags(heap()->incremental_marking()->IsMarking());
        HeapObject object = page->GetObject();
        heap()->StartIncrementalMarkingIfAllocationLimitIsReached(
            heap()->GCFlagsForIncrementalMarking(),
            kGCCallbackScheduleIdleGarbageCollection);
        if (heap()->incremental_marking()->black_allocation()) {
            heap()->incremental_marking()->marking_state()->WhiteToBlack(object);
        }
        DCHECK_IMPLIES(
            heap()->incremental_marking()->black_allocation(),
            heap()->incremental_marking()->marking_state()->IsBlack(object));
        page->InitializationMemoryFence();
        heap()->NotifyOldGenerationExpansion();
        AllocationStep(object_size, object->address(), object_size);
        return object;
    }

    LargePage* LargeObjectSpace::AllocateLargePage(int object_size,
        Executability executable)
    {
        LargePage* page = heap()->memory_allocator()->AllocateLargePage(
            object_size, this, executable);
        if (page == nullptr)
            return nullptr;
        DCHECK_GE(page->area_size(), static_cast<size_t>(object_size));

        AddPage(page, object_size);

        HeapObject object = page->GetObject();

        heap()->CreateFillerObjectAt(object->address(), object_size,
            ClearRecordedSlots::kNo);
        return page;
    }

    size_t LargeObjectSpace::CommittedPhysicalMemory()
    {
        // On a platform that provides lazy committing of memory, we over-account
        // the actually committed memory. There is no easy way right now to support
        // precise accounting of committed memory in large object space.
        return CommittedMemory();
    }

    LargePage* CodeLargeObjectSpace::FindPage(Address a)
    {
        const Address key = MemoryChunk::FromAddress(a)->address();
        auto it = chunk_map_.find(key);
        if (it != chunk_map_.end()) {
            LargePage* page = it->second;
            CHECK(page->Contains(a));
            return page;
        }
        return nullptr;
    }

    void LargeObjectSpace::ClearMarkingStateOfLiveObjects()
    {
        IncrementalMarking::NonAtomicMarkingState* marking_state = heap()->incremental_marking()->non_atomic_marking_state();
        LargeObjectIterator it(this);
        for (HeapObject obj = it.Next(); !obj.is_null(); obj = it.Next()) {
            if (marking_state->IsBlackOrGrey(obj)) {
                Marking::MarkWhite(marking_state->MarkBitFrom(obj));
                MemoryChunk* chunk = MemoryChunk::FromHeapObject(obj);
                RememberedSet<OLD_TO_NEW>::FreeEmptyBuckets(chunk);
                chunk->ResetProgressBar();
                marking_state->SetLiveBytes(chunk, 0);
            }
            DCHECK(marking_state->IsWhite(obj));
        }
    }

    void CodeLargeObjectSpace::InsertChunkMapEntries(LargePage* page)
    {
        for (Address current = reinterpret_cast<Address>(page);
             current < reinterpret_cast<Address>(page) + page->size();
             current += MemoryChunk::kPageSize) {
            chunk_map_[current] = page;
        }
    }

    void CodeLargeObjectSpace::RemoveChunkMapEntries(LargePage* page)
    {
        for (Address current = page->address();
             current < reinterpret_cast<Address>(page) + page->size();
             current += MemoryChunk::kPageSize) {
            chunk_map_.erase(current);
        }
    }

    void LargeObjectSpace::PromoteNewLargeObject(LargePage* page)
    {
        DCHECK_EQ(page->owner()->identity(), NEW_LO_SPACE);
        DCHECK(page->IsLargePage());
        DCHECK(page->IsFlagSet(MemoryChunk::FROM_PAGE));
        DCHECK(!page->IsFlagSet(MemoryChunk::TO_PAGE));
        size_t object_size = static_cast<size_t>(page->GetObject()->Size());
        static_cast<LargeObjectSpace*>(page->owner())->RemovePage(page, object_size);
        AddPage(page, object_size);
        page->ClearFlag(MemoryChunk::FROM_PAGE);
        page->SetOldGenerationPageFlags(heap()->incremental_marking()->IsMarking());
        page->set_owner(this);
    }

    void LargeObjectSpace::AddPage(LargePage* page, size_t object_size)
    {
        size_ += static_cast<int>(page->size());
        AccountCommitted(page->size());
        objects_size_ += object_size;
        page_count_++;
        memory_chunk_list_.PushBack(page);
    }

    void LargeObjectSpace::RemovePage(LargePage* page, size_t object_size)
    {
        size_ -= static_cast<int>(page->size());
        AccountUncommitted(page->size());
        objects_size_ -= object_size;
        page_count_--;
        memory_chunk_list_.Remove(page);
    }

    void LargeObjectSpace::FreeUnmarkedObjects()
    {
        LargePage* current = first_page();
        IncrementalMarking::NonAtomicMarkingState* marking_state = heap()->incremental_marking()->non_atomic_marking_state();
        // Right-trimming does not update the objects_size_ counter. We are lazily
        // updating it after every GC.
        size_t surviving_object_size = 0;
        while (current) {
            LargePage* next_current = current->next_page();
            HeapObject object = current->GetObject();
            DCHECK(!marking_state->IsGrey(object));
            size_t size = static_cast<size_t>(object->Size());
            if (marking_state->IsBlack(object)) {
                Address free_start;
                surviving_object_size += size;
                if ((free_start = current->GetAddressToShrink(object->address(), size)) != 0) {
                    DCHECK(!current->IsFlagSet(Page::IS_EXECUTABLE));
                    current->ClearOutOfLiveRangeSlots(free_start);
                    const size_t bytes_to_free = current->size() - (free_start - current->address());
                    heap()->memory_allocator()->PartialFreeMemory(
                        current, free_start, bytes_to_free,
                        current->area_start() + object->Size());
                    size_ -= bytes_to_free;
                    AccountUncommitted(bytes_to_free);
                }
            } else {
                RemovePage(current, size);
                heap()->memory_allocator()->Free<MemoryAllocator::kPreFreeAndQueue>(
                    current);
            }
            current = next_current;
        }
        objects_size_ = surviving_object_size;
    }

    bool LargeObjectSpace::Contains(HeapObject object)
    {
        MemoryChunk* chunk = MemoryChunk::FromHeapObject(object);

        bool owned = (chunk->owner() == this);

        SLOW_DCHECK(!owned || ContainsSlow(object->address()));

        return owned;
    }

    bool LargeObjectSpace::ContainsSlow(Address addr)
    {
        for (LargePage* page : *this) {
            if (page->Contains(addr))
                return true;
        }
        return false;
    }

    std::unique_ptr<ObjectIterator> LargeObjectSpace::GetObjectIterator()
    {
        return std::unique_ptr<ObjectIterator>(new LargeObjectIterator(this));
    }

#ifdef VERIFY_HEAP
    // We do not assume that the large object iterator works, because it depends
    // on the invariants we are checking during verification.
    void LargeObjectSpace::Verify(Isolate* isolate)
    {
        size_t external_backing_store_bytes[kNumTypes];

        for (int i = 0; i < kNumTypes; i++) {
            external_backing_store_bytes[static_cast<ExternalBackingStoreType>(i)] = 0;
        }

        for (LargePage* chunk = first_page(); chunk != nullptr;
             chunk = chunk->next_page()) {
            // Each chunk contains an object that starts at the large object page's
            // object area start.
            HeapObject object = chunk->GetObject();
            Page* page = Page::FromHeapObject(object);
            CHECK(object->address() == page->area_start());

            // The first word should be a map, and we expect all map pointers to be
            // in map space or read-only space.
            Map map = object->map();
            CHECK(map->IsMap());
            CHECK(heap()->map_space()->Contains(map) || heap()->read_only_space()->Contains(map));

            // We have only the following types in the large object space:
            if (!(object->IsAbstractCode() || object->IsSeqString() || object->IsExternalString() || object->IsThinString() || object->IsFixedArray() || object->IsFixedDoubleArray() || object->IsWeakFixedArray() || object->IsWeakArrayList() || object->IsPropertyArray() || object->IsByteArray() || object->IsFeedbackVector() || object->IsBigInt() || object->IsFreeSpace() || object->IsFeedbackMetadata() || object->IsContext() || object->IsUncompiledDataWithoutPreparseData() || object->IsPreparseData()) && !FLAG_young_generation_large_objects) {
                FATAL("Found invalid Object (instance_type=%i) in large object space.",
                    object->map()->instance_type());
            }

            // The object itself should look OK.
            object->ObjectVerify(isolate);

            if (!FLAG_verify_heap_skip_remembered_set) {
                heap()->VerifyRememberedSetFor(object);
            }

            // Byte arrays and strings don't have interior pointers.
            if (object->IsAbstractCode()) {
                VerifyPointersVisitor code_visitor(heap());
                object->IterateBody(map, object->Size(), &code_visitor);
            } else if (object->IsFixedArray()) {
                FixedArray array = FixedArray::cast(object);
                for (int j = 0; j < array->length(); j++) {
                    Object element = array->get(j);
                    if (element->IsHeapObject()) {
                        HeapObject element_object = HeapObject::cast(element);
                        CHECK(heap()->Contains(element_object));
                        CHECK(element_object->map()->IsMap());
                    }
                }
            } else if (object->IsPropertyArray()) {
                PropertyArray array = PropertyArray::cast(object);
                for (int j = 0; j < array->length(); j++) {
                    Object property = array->get(j);
                    if (property->IsHeapObject()) {
                        HeapObject property_object = HeapObject::cast(property);
                        CHECK(heap()->Contains(property_object));
                        CHECK(property_object->map()->IsMap());
                    }
                }
            }
            for (int i = 0; i < kNumTypes; i++) {
                ExternalBackingStoreType t = static_cast<ExternalBackingStoreType>(i);
                external_backing_store_bytes[t] += chunk->ExternalBackingStoreBytes(t);
            }
        }
        for (int i = 0; i < kNumTypes; i++) {
            ExternalBackingStoreType t = static_cast<ExternalBackingStoreType>(i);
            CHECK_EQ(external_backing_store_bytes[t], ExternalBackingStoreBytes(t));
        }
    }
#endif

#ifdef DEBUG
    void LargeObjectSpace::Print()
    {
        StdoutStream os;
        LargeObjectIterator it(this);
        for (HeapObject obj = it.Next(); !obj.is_null(); obj = it.Next()) {
            obj->Print(os);
        }
    }

    void Page::Print()
    {
        // Make a best-effort to print the objects in the page.
        PrintF("Page@%p in %s\n", reinterpret_cast<void*>(this->address()),
            this->owner()->name());
        printf(" --------------------------------------\n");
        HeapObjectIterator objects(this);
        unsigned mark_size = 0;
        for (HeapObject object = objects.Next(); !object.is_null();
             object = objects.Next()) {
            bool is_marked = heap()->incremental_marking()->marking_state()->IsBlackOrGrey(object);
            PrintF(" %c ", (is_marked ? '!' : ' ')); // Indent a little.
            if (is_marked) {
                mark_size += object->Size();
            }
            object->ShortPrint();
            PrintF("\n");
        }
        printf(" --------------------------------------\n");
        printf(" Marked: %x, LiveCount: %" V8PRIdPTR "\n", mark_size,
            heap()->incremental_marking()->marking_state()->live_bytes(this));
    }

#endif // DEBUG

    NewLargeObjectSpace::NewLargeObjectSpace(Heap* heap, size_t capacity)
        : LargeObjectSpace(heap, NEW_LO_SPACE)
        , pending_object_(0)
        , capacity_(capacity)
    {
    }

    AllocationResult NewLargeObjectSpace::AllocateRaw(int object_size)
    {
        // Do not allocate more objects if promoting the existing object would exceed
        // the old generation capacity.
        if (!heap()->CanExpandOldGeneration(SizeOfObjects())) {
            return AllocationResult::Retry(identity());
        }

        // Allocation for the first object must succeed independent from the capacity.
        if (SizeOfObjects() > 0 && static_cast<size_t>(object_size) > Available()) {
            return AllocationResult::Retry(identity());
        }

        LargePage* page = AllocateLargePage(object_size, NOT_EXECUTABLE);
        if (page == nullptr)
            return AllocationResult::Retry(identity());

        // The size of the first object may exceed the capacity.
        capacity_ = Max(capacity_, SizeOfObjects());

        HeapObject result = page->GetObject();
        page->SetYoungGenerationPageFlags(heap()->incremental_marking()->IsMarking());
        page->SetFlag(MemoryChunk::TO_PAGE);
        pending_object_.store(result->address(), std::memory_order_relaxed);
#ifdef ENABLE_MINOR_MC
        if (FLAG_minor_mc) {
            page->AllocateYoungGenerationBitmap();
            heap()
                ->minor_mark_compact_collector()
                ->non_atomic_marking_state()
                ->ClearLiveness(page);
        }
#endif // ENABLE_MINOR_MC
        page->InitializationMemoryFence();
        DCHECK(page->IsLargePage());
        DCHECK_EQ(page->owner()->identity(), NEW_LO_SPACE);
        AllocationStep(object_size, result->address(), object_size);
        return result;
    }

    size_t NewLargeObjectSpace::Available() { return capacity_ - SizeOfObjects(); }

    void NewLargeObjectSpace::Flip()
    {
        for (LargePage* chunk = first_page(); chunk != nullptr;
             chunk = chunk->next_page()) {
            chunk->SetFlag(MemoryChunk::FROM_PAGE);
            chunk->ClearFlag(MemoryChunk::TO_PAGE);
        }
    }

    void NewLargeObjectSpace::FreeDeadObjects(
        const std::function<bool(HeapObject)>& is_dead)
    {
        bool is_marking = heap()->incremental_marking()->IsMarking();
        size_t surviving_object_size = 0;
        bool freed_pages = false;
        for (auto it = begin(); it != end();) {
            LargePage* page = *it;
            it++;
            HeapObject object = page->GetObject();
            size_t size = static_cast<size_t>(object->Size());
            if (is_dead(object)) {
                freed_pages = true;
                RemovePage(page, size);
                heap()->memory_allocator()->Free<MemoryAllocator::kPreFreeAndQueue>(page);
                if (FLAG_concurrent_marking && is_marking) {
                    heap()->concurrent_marking()->ClearMemoryChunkData(page);
                }
            } else {
                surviving_object_size += size;
            }
        }
        // Right-trimming does not update the objects_size_ counter. We are lazily
        // updating it after every GC.
        objects_size_ = surviving_object_size;
        if (freed_pages) {
            heap()->memory_allocator()->unmapper()->FreeQueuedChunks();
        }
    }

    void NewLargeObjectSpace::SetCapacity(size_t capacity)
    {
        capacity_ = Max(capacity, SizeOfObjects());
    }

    CodeLargeObjectSpace::CodeLargeObjectSpace(Heap* heap)
        : LargeObjectSpace(heap, CODE_LO_SPACE)
        , chunk_map_(kInitialChunkMapCapacity)
    {
    }

    AllocationResult CodeLargeObjectSpace::AllocateRaw(int object_size)
    {
        return LargeObjectSpace::AllocateRaw(object_size, EXECUTABLE);
    }

    void CodeLargeObjectSpace::AddPage(LargePage* page, size_t object_size)
    {
        LargeObjectSpace::AddPage(page, object_size);
        InsertChunkMapEntries(page);
    }

    void CodeLargeObjectSpace::RemovePage(LargePage* page, size_t object_size)
    {
        RemoveChunkMapEntries(page);
        LargeObjectSpace::RemovePage(page, object_size);
    }

} // namespace internal
} // namespace v8
;
